You have to determined where you are going and how hard the water is. There are several maps online to help on that. I live in New Mexico and we have very hard water. States around us also are the same. Ours unit is 48000 unit. It good for 546 gals plus of soft water at 56 grains hardness. If water is higher in grains then less water, if lower then more water. Its all set up in the Head unit on the water softener. Every recharge take right at 10 lbs of salt about two hours to recharge. Last about 10 days at that hardness "Not these small box of table salt units" The salt cleans the resin , if not clean then they can not work. We have live here since the mid 80's so have deal with it many years. Very few places that we have visited have nice water. The key is the more resin that the unit is rated the longer you have good water. The resin last around 10 plus years and then you replace the resin.
I would say get as high as you can fit and go from there. I install mine in a closet above the wet bay and put the salt container in the wet bay almost under it so just hoses to connect. Has work that way since 2000