If I may add my humble opinion....
Out of all the tanks I have(ranging from 2-50 gallons)my 5 gallon is probably my favorite. It doesn't take up a lot of space, its cycled so I only need to preform weekly PWCs which take only a few minutes, and if I so choose I can divide it for another betta. If you have the option, I would say go for a 5 gallon and cycle it.
I hear anything smaller is almost impossible to cycle....dunno how true that is, but I know a couple people on here have full 3 gallon setups...I just wish I could remember who so I could ask if theirs are cycled or not >_<; But, anything smaller then a 5 gallon I believe will need some sort of cleaning more then once a week...
Anyway, for heaters the Hydor Mini Heater is great for something small....I have them in my 2 gallon critter keepers and they work alright. Though it didn't do well in my 5 gallon, just couldn't keep it warm enough. I'm unsure how well it'd heat in a 3 gallon.....though I'd probably try for an adjustable heater anyway, just to be safe, especially if where the tank will be will get cold....
Craigslist is a good place to tank shop too, I always see lots of great deals. I've never used it personally, though I agree about meeting in a public place and perhaps dragging a friend along with you. That'd make me feel more safe