I built a road bike last year and I am trying to build a mountain bike
now as I have some spare wheels and parts I could use to help build
it. I am mostly going to use this as a road/dirt road bike in winter
time but will do some offroad on occassion. I am currently looking for
an inexpensive suspension fork and I have noticed that some are disc
brake only which seem to have the best price. I don't really know much
about disc brakes.

Is there a link on how they work and how to install them? Is that much
harder?

I was planning on buying a fork with cantilever brakes but haven't
found one at a lower price to purchase but I see the prices on disc
brakes are alot more too so there may be an offset.

Also, I already have a MTN Bike with an older 7 speed setup. Is it
easy to find 7sp parts for a new MTN bike I am building now since I am
wanting to be able to swap wheels.

Thanks in advance

Matt p