1) I think I've found a relatively cheap pH controller which can trigger a small pump to offset rising pH. Would this help my cause or could it just make things worse?

Stinky of course is right about this. pH isn't really something to worry TOO much over. The nutes are buffered and will mostly stay good. There are a lot of other things to be considered. For instance, I am working with 91 degree temps and 85 degree water temps. Its doable, but I need to be constantly on top of balancing the EC just right with the rate of transpiration so that the plants don't suddenly get overdosed or locked out.

Today I came home and a plant had broken a major branch under its own weight and fallen over. If I didn't catch that today it would have been a gonner (and its gonna yield a quarter pound dry).

2) I've read about a thousand different (and often conflicting) opinions with regards to HPS vs. CFLs. I am planning a small/medium sized stealth grow cab where heat is likely to be something I will be fighting with. Can someone tell me if a 150watt HPS will be harder to deal with than a comparable (150-200 watts) amount of CFLs?

There is a lot of half truths about "alternative" lighting. First lets start with MJ using a maximum of 10K Lux. A 150 HPS doesn't produce that minimum so it won't be optimal, but that is OK. Enough floros can produce 10K Lux however at no penetration. HPS penetrates into the canopy and has a significant advantage. Also and particularly important is that while you can easily find a 3000K "spectrum" floro, it really doesn't provide much in the way of the other parts of the spectrum that flowering still uses like HPS.

I would still look closer at soil then hydro if you don't want to monitor every couple of days at least. But if you insist on hydro probably a single bucket waterfarm or something will work best for you. Or an Emily's garden DWC kit. They are both very affordable, perfect size for 150 HPS and have a little bit of safety net if the power goes out (or timer breaks, etc...).

Remember that is another killer in hydro: electronic timers sometimes don't recover from power outages, analog (motor) timers sometimes get weak over time and can't flip the microswitch. Shit happens in an instant that changes their world.

Change the title of your thread and hit us back as you design your system.