Welcome to AA!
I like the ideas you have and the placement of the tank is awesome!
If it were me I would go with a 6' 210 RR tank. I would stay away from the 4x4's. They are not a good idea. Here is the stand
template that I used when building for my 120.
As for lighting, if you are going full reef with SPS
corals or clams I would suggest a diy style lighting with MH
in pendants instead of a fixture. You have more options and it will save you $$$. If you are going to have only the face of the tank showing with trim around it then your lighting can be easily accessed in your fish room.
Take it from me the surface on which you set your tank needs to be perfectly flat and level. I went so far as to buy a slab of granite to sit my tank on. I'm not suggesting you do that but keep it in mind as you build your stand. With a 6' tank you will need good center bracing to keep things level and avoid bowing.