If you're serious about this boat, or another steel boat down the road, hire a marine surveyor who's experienced with steel boats and get the boat surveyed.
The surveyor will go over the boat in detail and afterwards will give you a tour and point out all the issues he finds. He should give you a written report as well, and give you an idea of the costs to finish the boat.
If he has an ultrasonic thickness gauge, he can measure plating thickness from the outside and detect thinning from internal corrosion.
It'll cost several hundred dollars. But even if you don't buy this boat, you'll learn an awful lot about what to look for in a steel boat.
Also keep in mind this is a home-built steel boat. The materials to finish it don't need to be gold-plated. For instance, people have used steel and aluminum pipe for mast tubes. Galvanized wire rope, cable clips and galvanized turnbuckles are about 1/5th the price of stainless steel rigging. When you size the masts and boom try to set it up so your sail dimensions will match that of popular production boat(s), especially fast boats where owners replace sails often. Then you'll have a ready supply of used sails at low prices.
One thing to check is designer support. There are "official" Bruce Roberts plans, then there are the plans Bruce Roberts says are pirated. He's actually creating a black list of boats built to these plans. His site says: "DO NOT get YOUR BOAT on this list as you will have a lot of problems in insuring, registering or selling your boat."
I've also heard that he doesn't offer any support to people who've purchased incomplete boats that were built to his official plans, unless the new owner buys a new set of plans from him.
Stuff to check out before you adopt the project.