Welcome to the board!
First off, I hope you mean a 1999 Z250. They didn't make a 25' model in 1990!
Second, the advice I'll give you is what I give to all potential Mariah owners. Most of the parts for these were custom made in-house at the old Mariah plant, so there are many parts that may prove to be incredibly difficult if not impossible to replace if they are bad. These are things like the molded dash gauge cluster, trim panels, storage doors, stainless bow rails, fuel tanks, fiberglass seat backs, and a few others. That being said, check those things out first. If they don't appear to have any damage, or if they're in good shape, then you're off to a great start.
Look for any internal fiberglass damage, or large cracks. If there are, seriously reconsider the boat. Fiberglass repairs are expensive. Check around the engine mounts too. Make sure there's no fiberglass damage around there.
Check the interior side panels (where the grab handles are located). Make sure they are snug and secure, and that there's not a lot of flex in the panel. My panel on the port side has come loose and flexes around a little bit in rough seas. I'm working on a solution to the problem, so when I figure it out I'll post it. Make sure the bolster seats are bolted down and secure. In the cuddy interior, the port side back panel (it's the one with the small spotlight on it) can work itself loose over time. Make sure it seems secure. The sink may seem like it doesn't work, but that's because it's a manual pump sink, and it uses a plastic bladder which may or may not be there, (it'll be located in the storage closet).
A few other things, if you'll be boating in high seas, make sure your docking lights are secured. They are inside the anchor locker and are secured to the housings by what looks like large hose clamps. Use a screwdriver and make sure they are tight. One of mine came loose during some rough water, and when the bulb fell off the housing, I had a 4" open hole in the front of the boat. Needless to say my cabin got soaked and I ended up taking on probably 20 gallons of water. The good news is these boats have great bilge pumps. That'll be another thing to check. Pull the rear seat forward, find the bilge pump below the engine (mine is black and yellow). There should be a float switch attached to it, which is a little square box. Feel around it and find the float, which is a small piece of plastic that moves up and down. Push it up and down a few times and make sure the bilge activates itself. If not, I'd suggest replacing the bilge pump ASAP. That's one thing you never want to be without.
The good news is, a lot of the parts can be replaced (aside the ones I've listed above). I was the one who has compiled the "Mariah Parts List" here on the site, all starting with parts from my boat, so it should come in very handy for you. Engine-wise, Most service centers can work on Mercruiser and Volvo Penta engines. Yes the engine hours are a little high, but if the engine's been maintained, it shouldn't be an issue, and you should be able to get around 300-400 more hours out of it before it's time for a rebuild. Just keep that in mind when it comes to their asking price.
If you've got any pics of the boat, post them, or PM the link to me, and I'll check it out for you. I've got a pretty good eye for spotting things that aren't right on these boats.
Good luck, and hope you'll become a member of the Mariah family soon!
Advice on buying a Mariah Z250
