IPads - Need your collective wisdom on what to consider?

I am an Android person and generally dislike Apple products.

However, for a work project, I need to acquire an iPad for testing and demonstrations. It will be fairly light use and only for work; my go-to tablet will still be my Android. I am comfortable getting a used, but comparatively recent iPad – but I don’t know what to get.

What are the earlier iPad models?

What sort of memory requirements should I be looking for?

Do all of them run the latest Apple operating system?

I don’t care about built in 3g or 4g – do they all have wi-fi support?

If I find a used one, what sort of things should I check to be sure that it’s in good shape? What usually breaks?

Any other advice is appreciated.