well if u do a bar in a good enough way as far as material, thickness, and mounting points whoever does the rest of the cage can just incorporate it in the cage
As for picking up a bolt in roll bar, sure you could do that and re-sell it later on down the road. They are better than nothing.
Guess what? You almost always have to do the main bar first. But here is the thing, you will still need support fot the bar, it can stand alone, but porper reinforcement is needed to work as a roll bar.
If you wanted to skimp you could just do the basic 6 points, skip the side impact and halo bar and any cross bars. It would be “ok” for track days. This would require 5 bars, 8 bends, 6 plates, and ~50 inches of wled total. I just don’t know what Lee will ask, or even if he will decide to offer this service.
the same requirements for a bar will work for a cage. A roll bar is min of 1 3/4 .120 wall, a cage only needs 1 5/8 .120, so a bar is slightly thicker. Many guys convert their roll bar to a cage, remember that for NHRA if you add the parts for a cage, a window net becomes manditory. The only drawback to adding the main halo later, is welding the top, normally you tack it in, then remove the front down tubes and weld the top, then push it back up, this keeps it close to the headliner and allows a higher seating position.