Cheers and welcome to BF.

My first action would be to make sure the current brake setup is well adjusted, has good calipers, brake pads, new cables and housings. When working properly, caliper brakes are perfectly capable of sending you flying over handlebars if you don't pay attention. This is probably the most cost effective option you have. Add fenders and a rear rack, lights and a lock and you have a commuter bike.

If however you have set your mind on disc brakes: unless you're inclined to do the work yourself you'd probably be better off looking for a new disc brake equipped bike (possibly with fenders and rack). Instead of welding stuff you can also have the current fork replaced to a disc capable fork, but things can escalate from there rather quickly as mentioned (also new brake levers, and if your brake levers are integrated with your shifters, new shifters...) Your bike shop can tell you their total cost for that upgrade in your case, but my guess is, it's not worth it.

--J