K, makes more sense now. Regarding your current rotors, measure the diameter of the rotor; the most common current mountain bike disc brakes use 160mm diameter rotors, but there are other sizes as well. The brake caliper needs to be mounted to fit the diameter of the rotor.
Here's what has to match:
Caliper needs to be compatible with the mount on the front fork. In recent years, things have standardized down to two types, post and tab mounts. If you could post a photo of your fork, we could confirm what type of mount you have (or if it's an older type of mount).
Lever needs to have pull ratio compatible with V-brakes/mechanical disc rather than the older cantilever brake pull ratio.
Any new rotor needs to be able to be mounted on your wheel! Current mounting type are 6-bolt and Centerlock, and the wheel hub type determines this. If you could post a photo of your front wheel hub (with current rotor on it is fine), we can see what type of mount you've got.
Check out the disc brake installation/service sections on the Park Tool website to get an idea of how disc brakes are mounted and adjusted. Assuming all the mounts that you have match current stuff, it's pretty easy to mount the brakes yourself.
See this for caliper mount types:
http://www.bikeman.com/bicycle-repai...4mm-post-mount
For comparison, a good quality single caliper and disc, with mounting hardware and instructions, cost about $50 at a USA discount mailorder shop:
http://www.jensonusa.com/store/produ...+Brake+10.aspx