More speculative answers here.
If the problem is that your R300 is spaced 126mm, you need to build a new rear wheel. My mechanic and I looked through QBP the other day for a halfway-decent rear wheel for my 126mm-spaced commuter, and there's only really trashy wheels. There are some perfectly good 126mm hubs still available, though, and whatever rim you want/will be compatible with your brakes can be laced on.
If it's 130mm and you have 7+ speeds, any new wheel is fine. If it's fewer than 7 speeds, you probably need to build a wheel on a 130mm threaded hub, or you could use this as an excuse to upgrade your drivetrain too.