I personally have seen just a 478 MB for checking Ram CPU...etc!
Anyway as a shop owner this is what you need at a minimum.
1.) PSU tester $10
2.) Mobo tested $20-100
3.) Winternals (or another boot up windows do-everything program)
4.) PC-Check (full hardware diag tool very good and cheap).
5.) Static liners, or static proof table.
6.) Network connections to all of the desk sections
7.) Full set of tools, including hex drivers for laptops.
8.) Collection of EVERY version of windows, for booting and repairing.
9.) Replacement PSU's.
10.) Spyware Virus removal Master Disk.
11.) at least 1 shop computer for cloning / ghosting HDDs.
12.) Most importantly. SKILL.
This is just a barebones list of what our shop uses. We are the best by far in the city im in, and I can say this because I have visited every other shop in town and spoke with the owner and techs.
Time = money. The more computers you can fix at the same time, the more money you make. Crappy service does not bring back customers. So go the extra mile and supply beter services. If you have any questions let me know.