to avoid clogging up my C: drive, I installed everything on a separate 2Tb SSD drive, and things are working just smoothly. MSFS is now almost 500 GB, it swells rather quickly.
I bought it in the MSstore, no real problems apart from after the last update when it wouldn't start anymore.
Which version to get? AFAIKS, the functionality is the same, it's just a few aircraft and airports more. The extra aircraft contained nothing that even vaguely interested me, and the airports were mainly large airports which I rarely visit anyway, so I bought the simplest version, and have no regrets whatsoever.
Flightsim.to is the place for freeware downloads, I have not bought a lot of scenery, since default looks so good already, but I was in desperate need for some interesting aircraft, default didn't really do it for me.
My favorites so far: Spitfire IX, Milviz Corsair, Dean's Stearman, the P-149 and now the DC-6..