choosing between Gen2 an Gen3 Hybrid
 

wondering if the batter in the 400h/450h should be a concern when buying used. looking at several with 50-95k.

Batteries of any kind do not last forever, I would recommend to pull out the calculator to weigh in expenses to compare with fuel only RXs (millage per year for city driving) if you find you will save enough $$$$ to compensate the cost to replace the battery down the road.....
then go for it.
Perhaps posting on the CL RX Hybrid section, you will get more users with hands down experiences to chime in.

We just switched from the 400h to 450h. The battery was always in the back of my mind but never really had any problems nor signs if it losing it's capacity. They really do tend to last a long time....just don't know how long.

The 450h is a nice improvement over the 400. And keep in mind, the 450h has a timing chain. The 400h is a timing belt needing to be changed every 100k or so. That was the reason we finally sold it at 170k.