1. 5 lb is "adequate", but get the biggest one that fits. 10 and 20 don't cost much more than 5, and they're cheaper (per lb) to fill. Maybe have a 5 as well, for backup. Tank can be in or out of the freezer as you describe, doesn't really matter.
2. No, you can run them all with one reg and a manifold. Of course, you're then compromising a little by having all your beers at the same carbonation level.
3. A new tank will run $70-100, a new 2-gauge reg probably $40-50. You can do simple picnic taps for ~$10 per, which is basically your cost for tubing, clamps, etc.. At the high end are the perlick/ventmatic faucets, which will run you $30-40 each just for the faucet. A tower (if you use one) can run anyhwere from $100 to basically whatever you want to spend.
4. I clean my lines with BLC and StarSan every time I kick a keg. Well, almost
. I've yet to disassemble my faucets and clean them.
5. The thermo should have a wide enough range on it (3-5 degrees) so that the compressor doesn't kick on that often. 5 degrees change in the ambient air won't budge the temp of your beer. (or you can put the probe into a glass of liquid, and set the tolerance tighter).
Hope that helps. Others will kick in with lots more, I'm sure.