Torque wrenches (like all other measuring instruments) are most accurate in the middle of their range. So you want to look at what fasteners you'll be torquing (cassette lockrings, seatpost clamp, handlebar/stem clamps get the most installation/removal on bicis), find the torque specs for each of them, and get a wrench with the appropriate range.

The drive and the socket are sized together also (to transmit the torque appropriately), but you can use an adapter within reason. The 4 mm and 5 mm Allen sockets you'll need for the clamps normally take a 1/4" drive, whereas the 1" hex socket for the cassette tool will take a 3/8" drive. (You'll probably see 1" sockets with 1/2" drive for automotive applications, but there's nothing on a bici that needs a 1/2" drive!) You can get a 3/8"-to-1/4" adapter to use the smaller sockets with your 3/8" drive wrench - should be fine for 4mm and 5 mm.