Dude! This is totally workable. Let me explain my iPad Mini experience:
I got the iPad Mini 16GB 4G AT&T model.
So I set up Google Voice, downloaded Talkatone to my iPad Mini, set up call forwarding to Google talk. Now I can dial out and receive calls for free with my iPod Touch/iPad/etc.
I went to Wal-Mart with an old T-Mobile Android phone (you can sell later) and set up Wal-Mart's T-Mobile $30 exclusive plan:
100 minutes
Unlimited Texting
Unlimited Data (5GB at 4G speeds)
Then I went to a T-Mobile store and had them cut my sim to a nano sim, popped it in, done deal.
iPad's are factory unlocked and a lot of networks are set up to use T-Mobile's new farmed 4G band. Check your area because all of San Diego has been refarmed. Even if you're stuck with AT&T you can set up Straight Talk which uses AT&T bands and has a better deal on data.
The bluetooth keyboard is a good idea but you can also pick up an Apple Wireless keyboard on Craig's List which gives you more of the MacBook look. Don't forget to jailbreak.