Get a boy if you only want one. Boys tend to be more sluggish *L*...the tend to rely more on human interaction as the have no mousey friend
Girls tend to always be on the go, and girls should be kept as 2 or more.
Get one that has been handled, if the assistant wont touch them dont get it, prolly means they are untamed biters, and believe me mice might be small but damn can the little shits bite!
Ask if you can handle the mouse proir to buying it, ensure it tolerates you holding it and patting (gently hold the tail as you do it though) if it jumps around like a looney, dont buy it.
Word of warning when handling unfamiliar mice, keep your palm skin very taut, taut skin is harder for them to get a hold of (and some mice can be like pitbulls, they hang on!).
If you can find, from a pet store, a friendly mouse, you will have one of the bestest buddies
soooo
1. Make sure its able to be handled (especially if you want it to be able to handled by kids)
2. Check no noises while its breathing (indicates URI's, which equal expensive to treat)
3. Ensure it is not hunched, got tumor growth anywhere, or gunky eyes
4. You want an alert mouse, but not one that practically climbs the walls when you go near it.
5. Get a good quality food (not made up of mainly sunflower seeds or corn)
6. Do not buy cedar for the cage/tank substrate.
7. You will need as a minimum, a house, a wheel, a drink bottle, food container and a toy of some type (empty toilet rolls etc are a hit *L*)