First of all, buy what YOU really want! Save until you can get it!
Secondly, you can't buy two rifles and swap bbls without a gunsmith doing the work for you. With that added cost, and the loss you will take on the extra gun, i don't think you will save a lot of money over buying what you want new in the first place.
It would take more time, but you could buy a used 700 Rem. in 7mm Rem. mag., and have it re-bbled in 257 Wby., but i don't know why? A 7mm Rem. mag., using a good quality 120 grain bullet will have similar recoil to the 257 wby., and kill deer just as fast or faster! PLUS, with heavier bullets the 7mm will take anything that walks north America!
DM