i agree with he suggestion to get a set of better calipers
and I would bet that just installing them on the front will be most of what you need
to be confident in your braking

you might try to simply replace the cable and housing with higher quality stuff
and get better brake pads
this will cost you about fifteen bucks per wheel
instead of the cost of new calipers
and certainly a lot less than the new wheels and fork needed to convert to disks

even if you get new brakes
disk or caliper
you should change the cables to something better than what came stock on the bike