I don't know about the USB thing. I have never (and probably will never) use it. However, my W205 has the capability. It just means buying another module. Alpine is kinda bad about making everything an add-on.
As for the differences in the cheap stuff and the more expensive stuff, features is just one part of it. You typically have a difference in the number of inputs/outputs, and compatability with other add-on modules (think of aftermarket support for certain vehicles). Also, some of the more expensive units will have better processing engines, DACs, stuff like that. It gets above my head as far as the internal technical stuff, but if you are into SQ then it's something to pay attention to. Built in crossovers and eqs are a big deal too. Also, for some, a 4v or 5v output may be worth more than a 2v output, and a lot of power from the deck can be more appealing to some, although to me neither is a huge deal. Certain companies are known for their research and development, design and engineering, reputation, customer service, support, warranty, etc. So among cheap units, I like an entry level Alpine over a Dual with more features, for example. Spending even more just gives you a better foundation, and normally better sound quality. Just remember that you can have great amps and speakers, but the head unit is where it all begins. But as you've covered, none of this really matter for you at this point since you just want something to get you by for now, right? Another option may be to look for a nicer used unit so you can get more for your money. I know I've given away and sold cheap some of my older units. I don't have any left right now, or I'd make you a deal on something.