Q: Difference between older/newer 27" Thunderbolt Displays?

I need a 27" display for my 21" iMac.

I had an old 30" cinema display, but I just didn't like it.

Someone locally has a first model/generation 27" LED Thunderbolt Apple display for $400. It's in the box, like new.

What are the differences between the early Thunderbolt models and subsequent models?

Is it the same display or different?

Is $400 a good price for this monitor? Will it hold its value?

The more research I do, the more it seems like it just isn't worth it to save a couple hundred bucks and get a cheaper monitor.

I appreciate any input!