The only potential downfall that I can see to white tubing over black is that if it isn't opaque enough, you might have crap growing inside of it. I had a nightmare with air bubbles when I was running a small, self-topping water-only system because our municipal water supply has a lot of entrained air in it. I went to great lengths to cure it, including grading all of the plumbing and putting in a solenoid valve to purge the air out automatically. Since I went to the upper-lower reservoir system, it has no longer been an issue because the water has plenty of time to outgas the entrained air. Do NOT run an air bubbler or pumps continuously. I would suggest that you carefully figure out the fittings that you need and then double or triple the number - I went through numerous revisions on my plumbing until I was satisfied, and the fittings were always the stumbling block to doing what I wanted.
I think that elevation should work fine. Something to keep in mind is that it is the water level that determines your head pressure, not where the bulkhead fitting is placed. Another thing that may impact your design is that the Blumats aren't happy with pressure fluctuations. If you have a tall, narrow reservoir, the pressure to the spikes is going to change substantially more than if you have a shallow, large-diameter one if you allow the level to drop much. This is another good reason for the upper-lower reservoir design, where a simple timer can keep your feed reservoir continually topped off within a few inches.