I have had several iPads. Buy the model with the largest storage capacity that you can afford. They are neither upgradable nor expandable (no memory card slot). You will use up memory faster than you expect!
The micro SIM is merely the latest and smallest mobile communications industry SIM card version. What SIM type the device has is of little or no concern to the user.
All devices that communicate over a cell provider's network have a SIM card, which identifies the device to the cell network. The SIM is only present on iPad models that have the mobile communications capability (AT&T or Verizon). It's not used, nor is it even needed, for communication via Wi-Fi - only for communication over a cell provider's network.
If you want an iPad with mobile communications capability, you must buy one with the AT&T or Verizon feature. It cannot be added later. All iPads come with Wi-Fi capability built in, though.
Hope this helps.
Ken