The 6.5x47 actually has very good barrel life & from all reports is capable of incredible accuracy. The setbacks are the initial reloading costs, factory ammo is, well.... rediculously expensive, you are going to want to reload & since Lapua is known for being the best brass on the planet you have that going for you. The 6.5 bullet diameter is starting to catch (again), with new offerings from several manufacturers, you're in good shape. You should be able to utilize your .243 bolt & magazine, which will reduce overall cost.
The 6.5x47 won't have the "lazer" like trajectory (which is why it will have far more usable bbl life than a 7wsm), but should offer what you need to make your 600yd shot. (very close to 260rem performance).
The 300wsm will also offer longer bbl life than the 7wsm. Either wsm will carry "killing power" at much greater distance than the x47 at the cost of bbl life, powder & recoil. As stated, this will not fit your .243 bolt, opening it up is possible but in respect to cost, a proper bolt from PTG w/better extractor (read M-16) is far easier & an a much better overall quality product. The WSM, when utilizing the high BC projectiles will also be MUCH longer than originally designed, you will have to install an extended magazine (or DBM) which will also increase cost. (unless you run single shot)
Hopefully that helps muddy the water a little