The Ipad 2 Ipad 3 and Ipad 4 are all 99.9% the same size so there are a lot of cases "backwards compatible" for them. The Ipad Air however is different altogether and you will need to buy a seperate case for it. As far as keyboard cases go the logitech cases in my opinion are the best - they are also actually the most popular.
You may find that they are slightly more expensive than a similar looking cheaper model but that is because they are generally better made. I would personally recommend the Logitech ultra thin keyboard cover or the Logitech ultrathin keyboard folio case if you want a little bit more protection. They are made from Aluminium and are brilliant.
Logitech also have a Fabricskin keyboard case which I really like because its so stylish and unique but it will not be everyones cup of tea. The keyboard is "built inside" the fabric case so the keys are flat but it looks really good in my opinion. The fabric skin case is harder to type fast on but it is spill resistant. Worth looking at.
Although the Zagg cases are quite popular and look good I find that they are not the best made cases and a lot have worn quite soon. I did a list of the best keyboard cases on my site. mrproductreview.com