Bought my latest dry suit (DUI TLS350 Signature Series - measurements were dead on with mine) and my scooter (SS UV26 w/ UV18 hull) under the same circumstances. The DS had 3 dives on it (the only wear I could find was where the guy had wrote his name in it) and the DPV had two dives on the short tube; got both for a song and a smile (got to love some folks out there.)

If you are looking to save cash, my advice would be to skip the DS class. Instead hook up with someone that dives dry for advice and pointers and take the time to get used to the suit. Not that the classes ares totally useless, but there just not all that much to learn about diving a DS that cannot be done with an average level mentor. Inflate just enough to relieve the squeeze, learn to raise the exhaust valve so it can vent prior to ascending, and it really is not that much difference from diving heavy neoprene. Start off in a pool or shallow water just to get the feel of it and gradually go from there.

Undergarments are going to be a personal choice, depending largely on the water temps that you will be diving. Thinsulate is nice and toasty, but it is also expensive and does not take to washing well (read, get used to replacing it or learn to handle the stench.) A lot of folks find cheap thermal wear through regular sporting good stores (if it is marked as dive gear, the price automatically goes up given our limited market share.) I know a lot of FL cave divers that are more than comfortable wearing cheap polypro from Walmart, while others freeze unless they have on a set of 400gr thinsulate. I would try the cheaper stuff first and go for a layering effect; by all means I would avoid buying used undergarments unless you know the seller well (that's just me though, several go this route and have no complaints - btw, I have a set of XL Moby's 400gr thinsulate that I'll make you a good deal on {too hot for me}.)

You say boots, so I am assuming the suit has neoprene socks rather than attached hard boots. If this is the case try a pair of converse high tops (Chuck Conners All Stars); they work every bit as good if not better (and fit into fins better) than the rock boots at a fraction of the cost. Hoods again will depend on what you are comfortable with. Typically dry suit hood will not have the flap at the bottom since you have no place to tuck it; this does not mean that you cannot use one or cut the flap off of one that you already have.

Hope you enjoy your new suit.