It really depends on how old you find acceptable, luxury level, and price range. Adding base plates, braking system, tail light wiring, etc. will cost you at least $1,000 with DIY install and buying a lightly used relatively new braking system, new and pro install will run $2,500+.
Personally if I were looking for a light basic, grocery getter, and did not mind having a manual transmission, I would take a strong hard look at the Chevy Spark, I spent a week living with one as a rental car about 4 years ago and was impressed with it. They are light, cheap even when new, and have a surprising amount of space. The only down side is 0-60 in eventually, I suspect my motorhome could beat it in a drag race, or at least tie it.