I wold not recommend a 20" barrel on the .243 WSSM. The best bang for you buck for shooting the bullet weights that you have mentioned would be a fluted 22" barrel. Depending on what bullet and powder, you will get between 3200 and 3450 FPS out of a 87 grain bullet fired from a 22" barrel.
The WSSM uppers will fit on any standard AR-15 lower receiver with out any modifications. The only receivers I have ever had issues with have been the Colt receivers with sear-blocks.
The magazines are modified in several ways, and I also replace the existing follower with a modified Magpul anti-tilt follower. The feed-lips get opened up a little, the magazine catch slot also gets raised a little. A modified magazine comes with each upper, and extra magazines are always available.
There is a very fine line between "Hobby" and "Mental illness"