Can you send us a pic of the front fork? The mounting bracket (if it has one) should say disk diameter on it. If there isn't one, I would say it would be 180mm (It *could* be 160mm as it is cable). If you can't tell for sure, simply measure from your dropout (bit where the axle sits) to the top of the pad, you will soon know if there it 20mm in it! You can use any type of disk you fancy, but as with cars - put a new set of pads in when you stick a new disk on otherwise you will be very disappointed, especially with cable disks (they are actually fine when set up well).
As for wheels, you need to make sure that you do get a rim with a machined (braking) face, 99% of after market rims for MTB are disk specific now. See how the profile is for disk rims...
Whilst V brake rims have that lovely machined brake face around them...
You should be able to pick up wheels easily, plenty of random odd wheels on ebay and who wants non-matching part vbrake part disk brake wheels (you do, that's who!)
Crack on, get out riding asap!!