I think they're a good company, they're not hiding the fact that batteries are used. For folks who know what their doing and how to work with cells I think they're great. That deal I linked specifically won't work for me and super appreciate the help of folks here to figure that out.
You will get many opinions on this forum to consider before making an informed choice. It is wise to do what you are doing right now...ask questions.
Just a couple of observations to add to the list:
There is nothing wrong with Battery Hookup. Like you said, they don't hide what they are doing. They provide a valuable service that keeps material out of landfills/hazardous waste sites, and in many cases allows people to build what they want. I do find it frustrating that they rarely answer their phone.
I wouldn't make the ASSUMPTION that keeping your 24v inverter is cheaper than changing it out for 48v. Keep on doing your research, and make a lit of all the components that you will need to have a properly constructed bank, including fuses, breakers, cabling, etc. Do it for 24v and for 48v. You may find that your assumption is incorrect. I have a crap load of both 24 and 48 volt modules, and recently did a quick and dirty comparison, and it was cheaper to go with 48v. Another thing to keep in mind is if you can actually get the components you want, with all the shortages, price gouging, etc.
It can be way harder to tap, and reconfigure modules than you might realize.
BMS'es are expensive, and time consuming to install.
Read up on technical papers on lithium chemistries, and you will find that there are advantages and disadvantages for each of the chemistries. Most people have jumped on the LifePO4 bandwagon, but there are many backup systems successfully using other chemistries too.
What is your time worth? There are a lot of very nice, modular units coming out now that are ready to go out of the box, and are very price competitive.
Those Samsung modules you originally considered are beautiful...and would likely make a very nice golf cart battery, EV conversion, or similar. I would not use them as you described in your original text.
Don't be in a hurry...keep looking and learning. Best of luck...