For travelling, I bring along the D700 with a 24-70/2.8 or a 50/1.4 mounted and ready to shoot. I attach the MB-D10 only when I expect to do heavy shooting. I prefer this over the D3 for the following reasons:
1) I can go light, i.e. without the MB-D10 when not needed whilst the D3 is always the 'big' D3.
2) I can use the built-in flash for fill flash whilst in D3, I MUST bring along another Speedlite.
3) I can use the built-in flash as the commander and control other remote Speedlites, whilst on the D3, I'll need another Speedlite to act as 'commander'.
Get you FX body first, e.g. a used D700 and you'll start enjoying all FX possibilities. the D700 can use both your existing DX lenses and any FX lenses that you further purchase. Over time, you gradually phase out your DX lenses and replace with FX lenses.
Hope this helps.
Fred