I just got the 5 Stage Plus a few days ago and I'm glad I did. I think the inline TDS, flush valve, etc., are well worth it. However, you can always add in the other stuff later on if you can't wait or want to see how it goes. They have videos about adding stuff after the fact, not only to their systems, but to any RO/DI system. Since they assemble these in house, I'm sure they will have more soon. BTW, these things are solid, scream quality, and they are well packed for shipping. I'm more than impressed. I did a lot of shopping before I decided on BRS. Another plus for the plus is the flexibility of having all the parts for temporary and permanent installs. I'm running temporary until I can decided exactly what I want to do more permanently.
P.S. I didn't know about the group buys, but I'll be in on one in the future now that I have a BRS RO/DI system. (I'm not doing saltwater, but am on well water and will use it for my freshwater tank, hydroponics, and the R/O water for drinking and cooking uses).