I believe that flushing is done because there is salt build up inside the plant from nutes, it can cause lockout. I have been told that certain nutes need to be flushed on a more regular basis than others.
I understand the back asswards thing, it's happened to me a few times too. Trying to find s substitution will always leave you with a substitute that is not as good as what you will need in the end. Go check out the grow journals and note what lights are being used.....
I hear what you are saying, but unless you think that this will be the only time you ever grow, I would wait and save for the HPS. The 70 watt light just isn't worth it, IMO
That's one way....
My plan is to wait until I see my first amber trich and switch out the water in my res to plain water for at least a week, up until I see a satisfactory amount of amber trichs. Keep in mind that I use Holland's Secrect and you don't have to flush as long as some others.
It's actually "HPS" standing for High Pressure Sodium. Yes, they need a ballast and fixture to run, are you only finding the bulbs?
Relocating to a smaller area is a great idea, make sure you have a fan and are monitoring the heat.