First congratulations on the new boat !
There are a couple of versions of the 25XP (25XP, 25XPA, 25XPB). There should be a manufacturers plate on the top of the engine that indicates which model you have. Silver Cloud
is the local Universal Dealer and they can tell you the proper filter for your engine (don't assume the one that's on there is the right one).
There have been numerous discussions about using automotive filters on marine diesels. Bottom line there are differences; there've been endless threads on whether those differences matter. I weighed the $4 bucks saved on buying 1 filter per year against $10K for a new motor and it didn't seem like a good idea. YMMV
Your diesel fuel is probably OK. If you really think it's suspect drain the tank over the winter and add fresh fuel. It's an 18 gallon tank so it's not a huge expense.
For the little bit of diesel I used motoring out into Barnegat Bay from Forked River I'd top her off with road diesel out of a 5 gallon jug and add a little biobor. I never had a problem.
There are two things you absolutely SHOULD do on a "new to you" Catalina 30 -- check the tank for debris and remove the tank pickup screen.
Pull the cushions out of the aft berth, the tank is under an access panel. Four screws hold the fuel gauge sending unit in the tank. Remove the screws and the sending unit and shine a bright light into the tank so you can see the bottom. It should be bright and shiny, if the bottom is covered with black debris you'll be cleaning the tank this winter.
Next remove the fuel pickup tube from the front of the tank. When you get it out shine a light through the tube. If there is a metal screen in the tube it will be clogged with gunk and you'll need to remove it by pushing it out with a dowel. Drop it in the trash. Your fuel filters will remove any debris pulled up from the tank. The screen was a bad idea, over time it clogs with debris and can cause rough running and stalling that is a ***** to diagnose. Ask me how I know...
Be careful screwing the pickup and sending unit back into the tank as it's easy to over-tighten them and strip the threads. If the sending unit gasket looks questionable replace it. There's nothing worse then the smell of diesel in the cabin.
Your profile says you're on Barnegat Bay. Dave at Silver Cloud in Forked River is a Universal dealer and will have all the parts you need. On the C30 there are two fuel filters, a secondary fuel filter mounted to one side of the alternator on the settee wall and a primary on opposite side of the engine (same as the oil filter). The fuel system is self-bleeding.
Since the boat is new to you, you'll also want to pick up an impeller kit for the water pump while you're in there. Two different pumps were used - Sherwood and Oberdorfer. Look at the pump on the front of the engine to determine which one you have. There were a couple of different alternator belts used on these engines so don't go by the parts manual, write down the number of your belt for reference.
Depending on whether you're on the upper or middle bay you might want to check out Bob's Square Deal on Rt 9 in Lacey Township. He's always less than West Marine and the marinas on parts.
If you don't have the manuals for your boat just Google the model number. Also check out Tech Wiki ? Catalina 34 International Association
. This is the Catalina 34 owner's site, but most of what's there applies to the C30 and it's a great reference.
Best of luck with her,