Well, my first advice is to get into doubles and dive them at home as much as you can between now and then! See if you can hook up with somebody who tech dives at home, who can give you some feedback on your buoyancy and trim, and practice some emergency procedures with him. This will give you a huge leg up on your cave class. A good cave instructor won't rush you through the class -- They'll make you stay at one level until you're comfortable and competent, and having to stay in OW to get basic skills down would not make you happy during your three weeks, I don't think
It sounds as though you will be using your instructor's equipment -- regs, lights, reels, etc?
I WOULD bring an O2 analyzer. If you're going on to Full Cave, you should do some O2 deco, and the fill stations in MX do not have analyzers.
I did Intro in a 5 mil suit with a 2 mil hooded vest, and it was barely enough -- fine for the dives, but not enough for the debriefs in the water. I'm going dry for Cave 2. My husband did Full Cave in a wetsuit, but I don't know how he did it.
It's also not a bad idea to bring a bunch of Pepto Bismol tablets (or scrips for Cipro and Lomotil and Phenergan) as, each trip we've made, at least one person gets a bit of tummy trouble there.
As I'm writing this, I'm seeing the floor of our condo, strewn with regulators, backplates, lights, voltage meters, reels, spools and all the other bits and pieces of cave diving
Oh, and I'd DEFINITELY buy a thing of wetnotes, even if your instructor is going to want you to use a slate. Drawing maps of the dives you are going to do, and then annotating those maps on deco or after the dive, makes for a VERY nice reference for your next trip.
Oh, and put toilet paper in your bag when you leave each day. Many of the outhouses don't have any. (Some sites don't have outhouses
)
Who's your instructor going to be?