The Star is a decent hotel, cheap and good location. Its on my list of places to book when I get around to it. Gets a pretty decent rating on various sites I've seen.
The other options is this Sun Route Hotel which is around the same price in the same location, maybe a little more expensive.
These are just Shinjuku hotels though. There are plenty of options outside of Shinjuku and many of them are better hotels for the money. Also Shinjuku is nowhere near autosalon really.
Shinjuku and Shibuya are good for being central to a lot of different things, and the nightlife it good.