gel battery?

Depends on your definition of 'average EV'.

The 100Ah cell size will be very limiting to range and performance unless you run a higher voltage system to compensate. I personally wouldn't touch these considering that for twice the price I could get lithium cells which will outperform these significantly in both performance and range(no peukert effect). Putting a bunch of gel cells in series is also something that will need some care because if any of the cells gets overcharged it will develop a permanent void inside the cell and you'd have to remove that battery.

How many, 12 at the very least. If you are serious about using these specific cells though, you might want to post your range and performance goals because 12 probably won't meet them and going with more and you might end up with an expensive controller for a cheaper chemistry batteries that can't handle the performance of that controller, which in my opinion, is a terrible false economy.

If you want lead-acid, AGMs are superior to these in performance for probably roughly the same price as these gel cells.

I use Deka Gels in my Yugo EV. 12volt 24 series. 72 volt system.

I like them, but have never tried AGMs.

Overcharge is not that big of a problem. They are supposed to be charged to 14.4. Mine go to 15.5 with no damage on a daily basis.(sometimes 16.3)

They don't need balancing. They charge up just the same, whether it is with a 20 amp pack charger or individual chargers.

They will outlast FLAs by a long ways.

A couple of mine are 5 years old, a couple are 2 years old.
2 were used in a wheelchair for 2 years, then they sat in a garage for another year or so without any charging, before I got them.

They came up fine and have had no problems with them. I have added a couple new batteries last year, yet they all act the same.

I plan to keep using them, as the price is under $150.00 each.