The power is included in the rent
Be careful with this statement.
Meaning do research about your rent terms, if you have a maximum power cap per month. That's what would get you on something like this.
Currently, I'm not paying anything as far as power costs (which is nice), but what would be a good starting rig? Would several USB ASIC's be better than a single large unit?
well anything will work then. they get very hot though
as a new guy just buy the best $$/GH RATIO and u will do fine i think
remember some use ALOT of power and need special attention to power outlets cables and the like
Would this be profitable enough to add another unit every few months or so? I've seen a lot of people using various setups and saying that they get only a few cents a day worth of bitcoins.
You should do $$ to GH, but if your rent has a max power cap I would say s3's would work alright for you.
Avalons are another good alternative and can be made quieter than an s3.
You will need to consider your heat, noise and power requirements tho.
A good place to start looking is the hardware section for used miners:
https://bitcointalk.org/index.php?board=75.0