If you don't care about 11 or 12 speed and the range of a 11-speed is sufficient, 11 is the way to go as it is cheaper, especially for wear items like chain and cassettes. I wouldn't worry about resale since drivetrains are wear items anyway and something nicer will show up anyway. What you enjoy will have the best value.

you can have 11 or 12 speed with HG or xD hub body. HG is cheaper (like the sunrace 11-50T 12-speed cassette) and it obviously depends on what your wheels have. xD cassettes typically are more expensive by default. they are CNC-machines out of one block or have some other expensive assembly (riveted etc.). The HG (and the new microspline hub) don't have that inherent cost added.

If you go SLX for the RD, you may consider the XT shifter since it is nicer and has double-shift etc. All the current shimano 11-speed shifters and RD are compatible to each other. Same way some people recommend pairing the GX RD with one of the XO1 (or whatever) shifter.
keep in mind RD may get damaged from strikes, so saving money on the RD makes sense while the shifter lasts longer and is responsible for how shifting feels. The RD just moves around regardless.