a few words:
1)do your research
2)take your time, the longer you take the more you will learn( I'm buying my 2nd HU in 2 months, 1st wasn't SQ enough )
3)don't take their first estimate, talk them down and be prepared to walk. Asking if there are any other shops in the area will tell you a lot about them.
4)BBB(better business bureau) it will tell you about complaints filed against the shop
5)what pro-rabbit said
6)ask things like: if they run all new speaker wire, if the wire is ofc or cca, what kind of rca's they use, type of fusing, what is the warranty on equipment and installation ?
7)do your research, take your time