Most of that I did not follow except to see that the water company has done the hard part for you. I did put in 3 big-blue filter housings. It was easy to do 3 while I was doing it. You don't have to select the ideal filter combo the first time. One of my filter housings is empty, because I had not decided what I wanted there. I also have a 20-5 micron spun polyester cartridge followed by a 1 micron spun polyester filter. Surprisingly I am on about my 10th month. I have new cartridges at the ready.
Big Blue 4.5 by 20 inch filters and housings are pretty cheap-- but I guess that is relative. I suggest you buy a replacement housing sump (the blue part), although you might be able to find a whole housing with sump for about the price of just a sump. I also suggest you buy a filter wrench, replacement O-rings, some food-grade silicone grease and some 4.5 by 20 filters. I will message you a source or two, but you can find others I am sure. I used polyester, because my water is from a well.
The O-rings are important maybe, but you don't know until you unscrew the housing. If the previous person put vaseline on the o-rings, they will have swollen and will not be reusable.
Carbon block filters do more filtering, but they have bigger back pressure.