1) There isn't that much range in available CPUs. Currently the one that most Android tablets are using is Tegra 3. Most people if you ask about Android tablets, will recommend one that uses it. (However, not all tegra3s are the same).
2) Like they say, it really depends on your needs. I'd say at least 512MB, though probably a gig is better. I think it's better to just look at the popular models and look at their stats. Probably a good baseline is the Nexus 7, which most people say has good performance. The thing is, especially on Android, a lot of the performance depends on the OS. That is why Android enthusiasts are so obsessed about getting the latest build of Android. Probably you should worry more, re: future proofing, about which version of Android is on the system, and whether the manufacturer is going to update it.
3) It depends on your needs, really. I've only used up about 1/2 of my capacity on a 16GB tablet, some people get by with 8GB. You can use the cloud for data storage, but then that depends on your situation. If you are going to do a lot of gaming, the games with better graphics will eat up a lot of space. SD card slots are getting less common, also.
4) Tablet apps are actually weaker on android. There are a lot of iOS exclusives. However, this depends on which specific apps you need. I would recommend looking at both the Android and iOS app stores.
5) Yes, they will all have wifi.
I would say either get the Nexus 7 (Android) or get an iPad. Android tablets have a lot of future-proofing issues because of the OS update situation. To be honest, I think tablets are moving so fast you can't really speak of future proofing. It isn't a mature technology yet.