Depends on what condition the timber transom and stringers are in. 70's and 80's Haines hunters are some of the best boats around but most will have been rebuilt at least once by now.

I was informed that when the hulls get to a certain age around 15 / 20 years they will need to be inspected by your insurer before they cover the boat , best to check with your insurance company before you buy

 Assuming we are talking about glass hulls only. Do some research, and you will find there have been serious hull rot issues with boats made by supposedly top-shelf Australian names, including the one mentioned above, at only 5 to 8 years old   I personally own a hull which was sold new in 1983, and which appears to be as solid as the day it was built. Drilling the transom to fit a new motor revealed only clean dry wood. I removed the galvanised (original fitting) coach screws holding the old Reelax seat bases in place the other day, and they came out of dry, hard deck with only the lightest amount of surface rust present.  But this boat would be the exception rather than the rule. It depends on a number of things

• Original construction--proper Marine ply (bondwood )used ? Manufacturers cut costs by using construction grade ply or even form ply. Unless you seal this against water ingress to a degree of perfection rarely achieved, it won't last long. Hardwood of any type has the same result.  if you want to argue this, just think about Ply hulled boats--crayboats used to be made of bondwood, and it was only painted, lasted many years in constant contact with water. Some manufacturers did not even seal the underside of the deck ply before fitting, for crissake.

•   Storage--boats left outside, uncovered, and exposed to rain can rot from the top down, rainwater is worse than salt water, and leakage can occur down into the hull. Inner hull bungs left in also trap water, and the resultant moisture and humidity will attack any badly sealed timber below deck. Some people will always  leave that bung in as a matter of habit , " so they can't forget to put it in before launching" 
• Fittings. ---Any drilling into or through the glass into the timber is a good start point for rot. Outboard mounting is the most common, the holes should be sealed and allowing to cure before bolts are then fitted using as soft sealant. Any surplus holes should be allowed to dry thoroughly, then the holes tightly timber dowelled with 24 hr araldite. Just using silastic/sikaflex is not enough in the long term. I have also seen a transom f*ck*d by the later addition of an auxiliary bracket--when the transom was stripped, it was the only rotten patch.
• The actual GRP it self is bloody near indestructible, unless the rot had been so bad for so long that flexing has caused the hull itself to crack. Forget about front bar talk about Osmosis, this only happens/happened in hulls left permanently in the water. Non-event for trailerboats. Gelcoat wil degrade if badly weathered, this can be rectified, but not cheap. If a boat has bad gelcoat cracking ,I'd just stay away.Fading is OK, can be cut and polished.

So where do you go from here? You have to find someone who knows their stuff for a hull condition report. You will likely need this for insurance purposes anyway if the hull is more than 20 years anyway. I can't make any recommendations for Perth in this regard, perhaps someone else can? And bear in mind that there are good businesses out there doing refurbs on older hulls all the time, it needn't be too expensive unless you elect to go the entire hull refinish road. Factor in these possible costs if looking at an older hull. Replacing a transom on a boat with a good low hours four stroke would be cheaper than putting a new four stroke on a later hull for a 2 stroke motor--and there are no guarantees that the 10 or 15  year old boat with the 2 stroke won't need hull work anyway.

HTH

All the goods points have been raised above so really all i am doing is re-capping.

- Look for spider web cracks, or impact marks. This can be a sign of damaged laminate

- Walk around the deck and feel for any soft points. This could be a sign of repaired damage, or rotten timber

- Get under the boat and push against the hull to look for any soft points.

- Have a look around the bung holes/outboard mounting bolts for condition. Is there much/any compression where the outboard mounts? (could be a sign of Rotten timber).

I purchased a hull which had sat uncovered for a year. The bung fittings had been removed and with water in the sump, the whole transome needed to be replaced. The boat ended up in the bin....

Per above, once the hull is over ~20 years, it will need to be inspected before it can be insured, and beyond that, every 5 or so years.

Some old boats are solid and structurally will be a very reliable and safe craft though it all depends on how and if it has been looked after.

 My boat was well over fifteen years old when I pulled her apart and she was buggered - some of the 20mm timbers were swollen to 40mm - full of water and just generally hammered - i liked her and I've had since new so I rebuilt her. it was more expensive than buying a similar size second hand but newer one but it was something I wanted to do and I didn't want to buy anyone else's problems; new donk on the back and she Goes a treat and I'm happy. Be careful with older glass boats if you don't know what to look for....:BUT if you get a good one some of the old ones leave the new ones for dead in regards to ride etc....in short no such thing as too old.....

in regard to insurance; mine is now 18 years old, never been inspected or certified nor does it have a builders plate (cause it predates the requirement) and I've never had an issue in getting insurance.....unsure why....but never been an issue (with SGIO  and was previously with CM).

 Probably be more of an issue with regards to insurance inspections, etc. if it's on an old trailer, but also hull rot is an issue.

Our hull is from the mid 80's, is in great condition, and rides nicely with a 2007 yammy 2st. As you mentioned get someone to check it properly and old hulls with newer motors go great 

 Swompa has added some good points. If you are looking at it yourself, look for cracking of the gelcoat at flex points on the transom, usually at the rear corners of the well. Also the compression mentioned. A rubber hammer is useful--get underneath and hit the hull, if the stringers are rotten enough to push the glass off the hull, you may hear rattles.

Everything like this is fixable, and it is often economically feasible to do the work, knowing that, if done properly, it is then good for life. As long as yoiu haven't paid too much in the first place.

My father and I looked at a fiberglass centre console many years ago at a reputable, now non existent boat yard. Laying under the boat, there were sections where you could physically deflect the hull.

Opted out of that boat because I would hate to think what sort of damage constant pounding would do the hull if you could just flex it my hand....

Thanks for the responses guys. The two boats I've considered with older hulls have been a 2000 Cruise Craft (sold) and a 2001 Caribbean and the other one is a 2008 Seafarer (powered in 2011)

 Go the carribean - awesome ride