As others have said, you need a water analysis before you buy anything. If you are on city water, you need to know the hardness. Your water company or their web site can tell you what the hardness is. If you have your own well, you need hardness, iron, pH, and if possible, the TDS (total dissolved solids), manganese, sulfates and chlorides at least. You can buy a limited type test kit at hardware stores, take a water sample to Sears, a lab or water treatment dealer or call one or more out to the house for a free no obligation test and demo of their softeners.
Big box store softeners are not the same quality as national brands or the softener sold by independent dealers. About 100% of the store and national brands are proprietary, meaning you have to buy parts and service from them. Independent dealers sell non-proprietary equipment and you can buy parts or service from many local or internet dealers.
Anyone with the desire to can install their own softener. It takes about 2.5 hours and is simple plumbing. Or they can hire a plumber to do it.
There are two parts to correctly sizing a softener, one is critical, the SFR (service flow rate - gpm). If the
SFR is not equal to or higher than your peak demand water use (gpm), the softener will not be able to remove all the hardness, iron etc.. The other part is capacity which is adjustable by adjusting the salt dose, which determines the salt
efficiency.
As to what softener.... I suggest any correctly sized softener using the
Clack WS-1 control valve. It has variable reserve and soft water brine refill while it is the easiest of all control valves to repair; a DIYer's dream. There are only three wearable parts and it can be totally rebuilt in about 20 minutes by anyone, even those that have never done anything like that before. It does not require any control valve specific special tools as most Fleck controls do, like the 5600 and 2510. And its fewer parts cost less.