In general, you want an aluminum receiver to keep the weight of the rifle down. The steel receiver was put into the Professional Hunter stock to mitigate some of the recoil generated by the larger calibers, however the aluminum receiver is capable of handling any cartridge in the R8 catalog. I have found that a steel receiver and kickstop is not necessary (for me) with a 375H&H or a 416RM. Above that, I'd at least want the kickstop to add some weight to the backside because the 22mm barrel profile adds so much weight to the front.
Your best bet is
BlaserBuds.com for a used stock. However if you properly prepared your timber stock, it would be perfectly capable of withstanding nearly any weather for up to a week. Of course some TLC will be needed on the backside of the trip...but it's still and option.
If it were me, I'd buy a whole R8 complete and explain to my wife that it was the only option I had to get a synthetic stock. Find one in 300WM and you will have matching bolt heads for the 375H&H. This would further reduce the number of parts you have, allowing you to sell the 30-06 and associated bolt head. It would also compensate you for some the money you spent on the complete 300WM rifle. I call that a "win-win".
