Isn't it amazing the difference a pressure washing will make?
We got a boat a couple years ago that had sat sealed up in a boatyard for six years, and the interior was better than what you've got, but not by much. The PO had left about a dozen empty bottles of different kinds of mold and mildew cleaners. Apparently none of them worked on a sealed boat for long.
The interior cleaned up quickly with bleach (after removing a pickup load of garbage). I bought an inexpensive solar vent and put it in the main cabin roof hatch
. We haven't had any problem with mold since then. Sealing leaky ports probably helped a lot with humidity, too.
So, solar vents are highly recommended.
If you go through and make up a shopping list of what you will need, then you can shop at your leisure when good buys come along. You can set eBay to search for particular items and email you when any come up.
Otherwise, welcome to the world of 'classic plastic'!