"IMHO, this would not be difficult to do in the M&P - basically a barrel, spring, and magazine swap, but somebody has to spend some money. An M&P in .22LR might be a better choice, although the price would likely be fairly high."

+1 A dedicated M&P makes more sense to me than a possibly finicky conversion unit. I have never liked the idea of field stripping one handgun to install a conversion unit and then having to reverse the process.