I think Erik summarises things very well, although if you could afford a D800, new housing and replacing lens then go for it! Mind you I would personally
never consider housing it in an Ikelite. As far as the speculation regarding the impending demise of DX, well it's just that; speculation.......
I have a D7100 now for topside nature images. I upgraded from the D7000 and I love it. Would love to house it and if I had the cash would do so.
Will the D7100 will be enough for you for a few years? That all depends on how well can you resist upgrading. There is no doubt it is an excellent crop
sensor DSLR capable of producing high quality images now and for as long as you continue to own and use it.