I'd skip it. There's not going to be a straightforward way to put a rear derailleur on it.
For $1500, it shouldn't take too long for a nice geared bike to show up on Craig's List. That's also going to be bikes that people sell through their shops on consignment, and some pretty nice '13 hardtails that are being sold new on clearance. So, make some phone calls.
If you're using price brackets to search faster, set your lower one to $500 or $600. You'd be surprised what people list their high-end, late-model hardtails for if they haven't been riding them.
Since you're willing to spend a reasonable amount of money, you should be able to find a nice bike without too much difficulty. Just look for one that is already set up pretty much how you want it.
"Don't buy upgrades; ride up grades." -Eddy Merckx