Replacing softener but not sure with what

I have a "Sears Kenmore Super Salt Saver 80 Plus" that is over 20 years old. I want to replace it. How do I choose a replacement that is equivalent to it? I have a 3 person household and very hard water. Is a water test required to get a proper softener? If I get a softener that is bigger than I need, to be sure it is adequate, is that a problem? For instance, Sears has a single cabinet unit that is 41,000 grains. This seems to be the largest. It is about $500 which is not to bad a price. If I only needed a 24,000 grain softener, for instance, it there a problem with buying the 41,000 grain model? Is the "grain" spec all that is really needed?
Here is a page from my manual that show my softeners capacity.....

my understanding (and I did this) is you need to get a water test so that you know how many grains of hardness to treat as well as any other quality issues with your water. I believe you are over-paying if you buy a system that is larger than you need.

I have four people in a four bedroom home with 16 grains of hardness and a wife who does a lot of laundry. We are on well water. I have a fleck sxt7000 sized for 48k. Hope that helps.

Looking at your specs, you really had a softener from Sears that was the equivalent of a 24000 grain unit. You had 3/4 cubic foot of resin. Capacity fluctuates with the amount of salt used per regeneration per cubic foot of resin.. 9 to 12 lbs is normal. If you jump that 9 - 12 lbs up to 35 or so lbs you can get more capacity, but the trade off isn't worth the salt usage.

If your buying a unit based on price, you will get about the same quality per dollar. The unit rafjr64 has is a good quality unit. The sears softeners are not.