Well if you were to buy a cheap gun like a JG or something, then be willing to do the research to find out which upgrade parts you want to buy in addition to the gun (tightbore barrel, gears, hopup bucking, bushings, piston, spring, some shims, etc), plus some gear grease and lubricants, then it could cost you either slightly less, or around the same (depending on which parts you get). Haven’t tried it myself though, but I mean to learn eventually.
Gearbox internals tend to be much cheaper than gun externals though. But in your case, I would much rather just pick something that performs well stock right out of the box, and wait for it to break later in the long run before attempting gearbox work. That way, you won’t have to mess around before you start shooting, and you’ll still be able to have room for upgrades in the future.
I personally don’t know much about Red Ryders, except that they tend to be gravity-fed, or so I found out last time I read up on them. I’ve heard things about the inaccuracy of Red Ryders, and that they’re mostly just for plinking cans and stuff like that. Metal BBs tend to do significantly more damage though, especially if they match the velocity of airsoft guns. I doubt that they’d be allowed in an Airsoft match, unless you’re talking about a backyard battle, and even then, Metal BBs pose risks due to their ability to break and/or shatter eye protective gear. Not to mention that they would hurt a lot more.
Pellet guns are only really good for plinking. When I fire them the pellets tend to squash on impact unless they’re hitting plastic (like Legos), in which case they either embed themselves into the plastic, or leave a nasty mark. Produces a pretty neat effect, actually.