Hello and welcome to the club!
Unfortunately much of the original Mariah parts were made in house, and now that the company is gone, parts like the seats and the dash are pretty rare finds. It has been suggested by many that if you want the original look, that you contact someone in fiberglass specialty (like a custom auto shop) to fix or rebuild your seats.
Same goes for the interior. Try a good coating of vinyl restorer/protector, but if it's really bad, you may just want to take it to a canvas shop and have it redone. Mariah used a heavyweight material in their builds. However, UV and time will do their damage, especially if left unprotected.
The Faria gauges have a limited lifetime warranty. If you have any problems with them you send them back to be repaired/replaced, for a nominal fee. There is a sticky thread in the original Mariah forum, wth links to all the vendors who sold the other parts in the boat.
Any other questions, just post up. There are a lot of helpful people on here.