I have used a Mech Tech. It's on my long list of items to pickup. Pistol Calibers in a rifle are GREAT training tools! It seemed fine and worked okay. Could be better, but the owner wasn't what I would call a saavy firearms owner... I did not get a chance to tear it down and look at it, so I really can't comment on quality or maintenance - Seemed accurate enough!
On the G21 receiver - Just have to keep your eyes open in the For Sale forums. Haven't seen a .45 receiver in the past so you may be off to your local LE Glock FFL to order one up. That is one way to do just the receiver... you can always pick up a used or refurbed G21 for a decent price. That would probably make the most sense to me.