A) geometry, brake type (discs/cantis)
B) Only if it has v brake/canti mounts. Road calipers will not do. So long as whatever frame you end up with has semi horizontal dropouts or a chain tensioner on vertical dropouts then you should be fine - but again, it will need v brake/canti mounts.
C) That question depends entirely on where your legs can take you and the technicality of the terrain that your friends are riding on.
A) geometry, brake type (discs/cantis)
B) Only if it has v brake/canti mounts. Road calipers will not do. So long as whatever frame you end up with has semi horizontal dropouts or a chain tensioner on vertical dropouts then you should be fine - but again, it will need v brake/canti mounts.
C) That question depends entirely on where your legs can take you and the technicality of the terrain that your friends are riding on.