The U2 is an USB miner, 2,5W isn't a lot of power. True though, it isn't efficient: the U2 is actually a revamped U1 with a bit better OC capability, but that's about it.
If you're planning on investing in ASICs, to get the best return, always go with the most expensive from a trusted company Mostly, only those are worth it to invest in.
On the other side, I have 10 U2s that I used for a month or two I could resell them at a bargain price (they're eating dust now anyways), but only if the OP is from Europe too
How much would you be looking for per unit? I live in the UK, so I am in Europe