Just bought series 7 slate last week. This one can run Windows 8 perfectly fine. I use the developer preview overall it works considering this one not even beta yet. I can tell you that if you need to buy it right now go ahead and do it but if you can wait then it might be more choices in the future.
From my experience using this tablet, I loved it.
PS. About the SIM card slot I have slot cover on the right below rotation lock button. I couldn't pop it up so not sure if mine has 3G or not. I have the 128GB model that comes with keyboard and stand.