+1 on the Mantis, of course. Right now, it's possibly the best possibility for starting out in modular. Not only does it give you 208 hp of space, the power supply is quite good, but you can also link Mantises to make a 4-row rig later on. And at $335, it's in the STOOPID-CHEEP zone as far as outlay's concerned. And Tiptop even makes a matching gig bag for it.
And the other thing Jim mentions is very true: starting with too small a cab will lead to the expense of another cab. Even if it looks like you'd "never" need another cab...you're going to need another cab eventually, so you may as well stall that as long as you can by getting something bigger and fixing the need for more cab space before it happens.