To be honest, if you know you are going to make the jump to FX. I would just bite that bullet. I pretty much blew my budget for basically this whole year on the D610. And I have absolutely no regrets. The kit lens actually is performing quite fantastic. And the true 24mm is outstandingly awesome.
And the only reason I say to bite the bullet as soon as possible is just because how I feel. My heart was telling me FX. So even though the D7100 is a great DX body....for me the D90 is still a great DX body. I didn't want two DX bodies. But that is just my thoughts on the subject. And it's just what worked for me.
If it wouldn't add another year to the "plan", I probably would, but I view it like this: As the side business grows, the D7100 makes a good pro-sumer product, then when I step up to full frame, it makes a high quality backup. If I dive right in to the D610, all I have for backup is the D3100, which is quite a change.
Never know, though. I might fall into a good portrait rhythm when the warmer weather starts up again, and perhaps...

I'd do a backflip to get one, for sure!