1) How important is something like a UA-5 that can handle mic power/gain/ADC?
A UA-5 will be a noticeable improvement over SP-CMC-4 > SP-SPSB-1 > NJB3. However, it is definitely not required to make a good tape. SP-CMC-4 > SP-SPSB-1 > NJB3 is certainly capable of making a very listenable pleasant recording.
2) Would that (UA-5) be something I could skip and add in later if I'm interested since the NJB3 handles gain/ADC although apparently not very well? ie run something like: SP-CMC-4 > SP-SPSB-1 > NJB3 and then run SP-CMC-4 > SP-SPSB-1 > ua5 > NJB3
a. if you are taping VERY loud shows then it is very likely that SP-CMC-4 > SP-SPSB-1 will not work; the SP-CMC-4's cannot handly high spl's. Tell us what bands you plan to tape and I'm sure someone can comment on whether they're too loud.
b. If the shows you plan to tape are not very loud then I recommend trying out the SP-CMC-4 > SP-SPSB-1 > NJB3. If after a year you are having a blast and you cannot get enough of taping then you know it is time to upgrade.
also, if you don't have soldering skills, then sp C4 > digimod UA-5 > jb3 is going to cost ~800, not to mention cost of cables/stand etc. I currently run this rig and it capable of making good to great recordings.
As far as your other questions about the at853 mics, I don't know the answers but I know someone will kindly chime in with the answers.