Any old glass boat will likely need a new transom, stringers, and floor - so just assume that your doing that. Beyond that, I would look for a boat that is mechanically complete - functional steering and controls, most gauges work, and a complete outboard or I/O setup. Yes, you will need to do some mechanical work - water pump impellers, bellows/bearings (on an I/O), new plugs and fuel filters, etc, but at least (hopefully) not having to go shopping for a full drive system.
You can replace/upgrade what you can afford, but at least you have a "complete" boat to start with.
Like mentioned earlier, avoid a cored hull, blisters, etc. If it's a bit scratched and weathered, new paint can fix that, but find a boat with a solid hull (other than the stringers/transom).
And get a boat that has a trailer included. Trying to find a trailer on it's own may cost more than buying a boat with a trailer - simply because getting rid of old boats is almost impossible once you sell the trailer from under them.
A free boat with no trailer will likely cost you more for the trailer...
And, make sure you get a boat you will enjoy once it's done. You're doing this for you, not somebody else.