I'll have to respectfully disagree with the notion that the motor is the most important thing to look at. It is no doubt important, but it is far cheaper to repair a motor than it is to do any major structural repair to a hull. That's especially true being that your step-dad is mechanically inclined. I'm a car guy myself, & I haven't found it very difficult to learn my way around an outboard.
I spent most of last year looking for an older used boat in decent shape. I was looking for something bigger on a bigger budget, but basically, I was after the same thing that they're after. I saw a lot of very nice running motors hanging off of rotting transoms, & I could have an outboard completely refurbished for about 1/2 the cost of a transom replacement.
I think they can find what they want in their price range. They may end up with an 18' instead of a 21', but they can certainly get something mechanically sound & structurally solid. I ended up with a 22' instead of the 25' I had set out after, but I'm much happier with it than I would've been with the clapped out 25 footers that were in my price range. In fact, a buddy of mine just recently sold a 19' Red-fin with a Yam 150 on it that would've been just right for them, & I believe he got somewhere around 4K for it.
For me, the key was patience. I started shopping around April & didn't buy anything until Sept. Don't rush into
any boat; have the patience to seek out the
right boat. That's what I did, & when I think back through all the boats I passed on, none of them would have worked out as well as the one I ended up buying.
Personally, I wouldn't own another boat with carpet. That's a matter of personal preference, but if they regard carpet as a negative, they shouldn't buy one that has it. They should be able to find a boat in their price range that doesn't have it, & they'll be money ahead for doing so because they won't feel compelled to pay someone to rip it out & paint the deck.
It's a buyer's market for used boats. There's way more out there for sale than there are buyers. There were boats on the market when I first started shopping around that were still on the market around the time I did the deal on my boat. In fact, the boat I used to own, the one with carpet, took me 8 months to sell.
As far as inspecting the hull, what you're looking for is a marine surveyor. They are paid professionals that make their living inspecting boats for buyers, insurance co's, etc. Typically, their work is focused on larger boats, & some of them will not survey a small boat. That's simply because there's not much of a market or much of a profit in surveying small boats. That said, I found someone that does, & he did a fine job. It was
Dixieland Marine Surveyors, & the surveyor I worked with was Frank Hawthorne. The service may seem expensive relative to the cost of the boat, but it's cheap relative to the cost of a major structural problem.
Two good web sites to check out are
The Hull Truth &
Yacht Survey. Yacht Survey is geared toward larger boats, but it has a lot of excellent info on boat design & construction rather than a lot of fluff.
Good luck!