I did Cave 1 in Mexico this year (same week as TSandM, actually, but different group) and while I was initially slightly concerned about their "lottery" system for assigning you an instructor, I couldn't have been happier with Fred. He was an awesome instructor, completely solid. I hear nothing but good things about Danny and Chris too, of course, so whoever you get will be great.
For me, Cave 1 in five days felt very doable. The days are quite long, but I never felt that the class was moving too fast for me to take it in properly. But I did have the benefit of having taken tech training (non-GUE, but quite DIR) the summer before; those coming into Cave 1 straight from Fundies might find it more demanding. MonkSeal, you're Tech 2, right? You should find it a breeze.
As for costs - yeah, Mexico is getting more and more expensive. Car rental, as TSandM mentioned, is quite overpriced compared to the US, although I can't remember now how much I paid. If your current credit card doesn't cover car insurance, it's worth upgrading to a gold card specially - the higher yearly fee is paid off in insurance savings within about three days.
Tank rental - $30 per set of doubles. Within Cave 1 limits and properly planned, you should be able to get two decent dives out of each set of doubles. So set aside $60/day for gas.
Accommodation - I stayed at a nice beachfront hotel in Akumal called the Vista Del Mar. If I remember correctly, shoulder season prices worked out to about US$110 per night single occupancy. [NB: Akumal is about 10 minutes drive from Zero G. That's something to think about if you're taking a course: it's nice to not have to schlep too far to get home after a long day.]
Health tips: I know she's a doc, but I think TSandM is a bit optimistic about the effects of the local strain of stomach upset. ;-) One of my Cave 1 buddies suffered with a dose of the local bellyache for a good week. [Kudos to him for making it through the course without any, erm... accidents.] Just bring a good supply of Immodium.
Oh - and ear infections are really common. I got one just after finishing the course and was out of the water for three days as a result. So use ear drops religiously at the end of every day. Zero G sells their own concoction that works well as a preventative. Otherwise, go to a pharmacy to get the heavy-duty stuff. The nearest pharmacy to Akumal is in Tulum.