Just a couple of ideas, but on the basis that this is homebuilt - why not see if you can lay hands on a copy of the plans? Useful for if you ever need to do any major surgery - but also you can check that it was built as designed, both underbuilt AND overbuilt (given the size adding weight will probably have a big impact on performance).
I think you said that their a few of these around, so why not have a chat with a couple of owners (ideally one who is also the builder) of sister ships - to get a lowdown on good and bad points, regarding the build, maintanence and performance / liveability. IMO would be worth spending the price of a few beers in "Consultancy Fees"
Also just to make the point (that would be true in this part of the world and I guessing in yours), and which I am sure you already know that she is a homebuilt steel boat (ie no "Brand Name") so no matter how much you spend on her their is a limit to what she will ever be worth.........of course only crops up when (if) reselling.
Although I have no first hand experinace with Steel Boats, my understanding (take with a pinch of salt!) is that everything can be repaired given time and skills (or money!), but that what you really
want is a sound hull to start with and this involves inspecting every nook and cranny - and checking the remaining plate thicknesses (both with a hammer and with "electrickery" - I forget the actual gadget name!!
).......I would also like someone who knew about welding to have a look at the welding (including on the out of sight areas) to see how much care / ability appears to have gone into her construction - although I beleive that a weld can only be truly tested by destruction.
I have also heard of cases where hulls can rust through in only specific spots due to "events" (electrickery? / wrong metals? / trapped bilge
water? / black magic? / bad luck?) which whilst of course not good would not mean the whole boat was fooked.