Many of the bigger rackmount UPSes have a big plug on the back to attach extra battery cabinets. I wouldn't get a big UPS without this feature. They'll let you hang whatever batteries you want off of the charger/inverter, as long as you get the voltage and charge rates right. (Obviously we are in warranty-voiding territory here.)
I have a couple of old Tripp-Lites, bought basically at scrap value because the internal batteries were dead, and they use Anderson SB175 forklift-charger connectors on the back. It works fine with just external batteries, giving you the advantage of being able to buy whatever SLAs happen to be cheap on the local market. (Though I'd definitely want to make sure all 4 were purchased at the same time since you're wiring them up together in a set.)
A user-replaceable battery that can be replaced in the way that an older Thinkpad battery can would be nice.
I'm looking at the options suggested [here](https://news.ycombinator.com/item?id=10831796) and [here](https://news.ycombinator.com/item?id=10831721), but I'm having a hard time finding APC battery availability in India.
Another benefit of using a standard inverter is that it is fairly lightweight so you can send it with the rest of the gear halfway around the globe and just source car batteries locally.