Gday Paul,i'll add my 2 cents worth. My dad worked in the boat industry all his life and i have worked, skied and fished for most of mine. Aluminium boats are lighter and more fuel efficient and need smaller engines to move them BUT, they ride extremely hard when compared to fiberglass boats, they are also wetter to ride in because most don't have the flared style of bow common on "glass" boats so when it gets a bit rough "tinnies" {aluminium boats} can be a very wet experience as well as bashing the #$%* out of you on the way home. The benefit of fiberglass is that they are dryer boats due to hull design and ride a hell of a lot softer that tinnies. I have a 15 foot Cruise Craft made in the 70's and i have done the repairs to it by myself and it isn't hard especially for someone with your obvious skills. The thing to look for in a Glass boat is the floor for soft spots because if the water has damaged the floor then the under floor stringers { wooden supports} are more than likely shot and while not impossible are quite a job to fix. The next thing to look for is the transom { back of the boat where the motor sits}, check it for signs of rot or soft spots because once the water is in it travels through the plywood rather fast, once again fixable but a reasonably big job, i have done both the floor and the transom on mine. I disagree that fiberglass is difficult to fix actually i find it rather easy. I would avoid any tinnie with rivets, Dehavelland used to make riveted hulls and after a short time they leak like a sieve, believe me i have owned a couple {they were cheap} Hope this helps and feel free to ask any questions. Seaweb has also some good experience with boats so maybe PM him to join the discussion.....
Mick