I don’t advise stuff like this. Its true that brake jobs are relatively simple - but any problems that arise will be in your lap. This kind of strategy can pay off but in the long run can be one that doesn’t actually save any money - in fact, it can cost more especially when you add in aggravation.
However, if you decide to go ahead, DON’T buy the cheapest brake stuff you can get. Get a known brand & at least the middle level of quality. When you buy ask whether there are hardware kits and get them. Ask the mechanic to use them. (Check the pads/rotors, though b/c sometimes they can come with new hardware bits). Do the same for brake grease.
I would just take it to a good local brake shop and have them do the brakes.