Scale is most important because it determines everything else. Make sure that the scale you choose has the equipment you want in the other 3 categories readily available.
HO will probably never be a problem. Other scales might be. The choice you make on scale has a HUGE impact on what the possibilities are for your MR.
My scale is already set (HO) so…Cost of product vs. quality…You can buy cheap stuff and fix it up yourself, or you can spend a lot of $$$ and not have to mess with it. ( I like doing things myself)[^][2c]