Okay,
With the idea being to upgrade into a reef later on down the line, I'm assuming you want to be able to keep SPS and clams which are the most light loving reef animals. They are also the most demanding of water quality, so if you can keep those, you should be able to keep almost anything.
Personally,
I would go w/ about a 30 gallon sump, DIY acrylic baffles in the sump to prevent bubbles from getting back to the main tank. I would place my heater and design a way to place the skimmer under the stand w/o taking up too much (in) sump space. I would go w/ about 1-1.5 (US) pounds of liverock per gallon of the whole set up(105 Gallons). I like more open areas in the reef/main tank, as opposed to the rock wall look of many tanks, so I would aquascape to my satisfaction and place the remainder of the rock in the sump. As long as the rock isnt interfering w/ you the water flow in the sump, or to the skimmer you should be fine. I would also take precautions to insure that you dont have a leak during a power outage. There are many ways to do this, let me know if I can help you out in that area.
Nick
