What I did was I bought a HD650 and ran it through a E11 amp. The SQ was nice enough for me to consider spending a lot of money on an amp and dac. Knowing I would want to upgrade anyway I bought an amp that was at the extreme end of how much I would want to spend. This way, in the long run I save a bit of money avoiding the typical buy and sell upgrade cycle. On the other hand, working your way up can be just as satisfying if not more, although I personally thought this was a better way for me.
I took a pretty big gamble, and in the end I am very satisfied with the results. A safer way to go about it would just be to find a used NFB-5 unit on the trade forums and try it out for yourself. If you like it...that's fantastic. If not, you can sell it at a very similar price than what you initially spent.
Once you have a certain amp/dac in mind, I would search for as many reviews as you can, and pick out the strengths as well as the flaws of the component. Sound signature is important, however other things such as heat, size, reliability, versatility with what you have, impedance matching, headphone pairing, price, aesthetics are all things to consider.
If you don't like to worry too much and are thinking you will upgrade in the future anyways, I would just opt for a 1 box solution and sell it if it does not fit your bill. Products such as the audio-gd ones are probably very popular at their price point and it wouldn't be too hard to find one used.