To do a total conversion to disk is not worth the money on a older bike. It's best to work with the current brake set up and put better brake pads on

To do a convert over to disk would range in the $$$ range if you're not doing your own work.

Here's two new bikes with disk brakes and one with dual suspension and one with front:
$300
http://www.bikesdirect.com/products/...basecamp_1.htm
$350
http://www.bikesdirect.com/products/gravity/fsx_1.htm

But really, don't put too much into a older mountain bike. They are not like road bikes which increase in value by how much you add on to them. An older mountain bike is a OLDER mountain bike, there is no money in upgrading them and they are not worth the money.

The guide to SS the bike.

Shopping list:
3/16 chain, just a cheap one
one ring taperred crank bolts
Bmx freehweel
Chain tensioner for 3/16 chain

What teeth count do I want for my freewheel?
Well consider what you're riding over:
Flat land/roads?
Hills/off road?
Mountain/downhill?

Do me a favor and count the teeth on the crank and give me an order like: 32/46/54
And then tell me what you want to use the bike for.

The # of teeth maybe labeled, mine are on the inside facing the frame