IMO if you can supplement the Nikon D610 would be a safer bet first.I always find it easier to sell gear then replace what you once had just in case you really miss the full frame goodness.It would kinda suck buying it all over again.
Thats what I'm afraid will happen. Because I know for a fact I'll miss the full frame sensor and the low light goodness. But the problem is that my budget will be small because if I was to sell my dslr now I could afford to buy a really top end mirrorless system..so I'll have to settle for something less which is why I've been eyeing the Ricoh GR and new one is coming out but doesn't seem to offer much new features so the older one probably can get for a good price. The only thing that bugs me about that camera is the lack of a viewfinder. So maybe I should consider a A6000 and a prime lens or a used Fuji X-E1 + 18mm f/2 or 27mm 2.8 or maybe the GX7 + 20mm f/1.7.
I don't know..the Ricoh GR is the cheapest and still a very good camera just lacks that viewfinder...errr.