It's tough figuring out what is the best solution would be. Personally, I would probably just get a ph testing kit at a home depot - it costs just a few dollars, and once your ph is set, it doesn't change very often (unless you add something to the soil). As far as as moisture, I use this soil moisture meter. It has worked well for me as it was in the Goldilocks zone for prices. Rather than costing $2000 as a professional piece of technology, it cost about 80. And it's not like the cheap ones, in that salinity/fertilizer doesn't affect it. As far as light, not sure what to say. It is obviously very crucial to a plant's growth and I know that certain colors of light are more essential than others, but I don't know of an affordable device that works well. Honestly, I think that the natural light of the sun makes the most sense...

urbngrdnr, since you say you are ''considering'' this purchase then you are probably looking for approval, encouragement and the like. Otherwise you would not be considering it, right? I have one that I bought at a yard sale for 25 cents. I really like it for determining when to water some houseplants. I doubt the benefit of a pH meter as soil pH is quite complicated related to cation ion exchange and that sort of thing. General Information on Alkalinity is a link to very basic pH related chemistry. cation exchange capacity discussion here.

The $5.00 ones work great. I've always kept one on the front porch and one on the back patio to test the moisture content in pots to make sure the water isn't just running through and not wetting the soil as it apt to happen sometimes in the hot summer time with soiless mixes that dry out completely. I just tried it on the lawn that was watered yesterday. The meter has a probe about 8 inches long. The top of it shows a four numbered scale from dry to wet. Pushed into the soil about an inch it tested on high side of dry. Pushed a little farther the indicator was in the middle between 2 and 3. Thrusted in deeper it the indicator went between 3 and 4 showing the soil was moist. I bought it at Lowe's several years ago for less than $5.00.

Are you leaving the moisture in place? If so, you will get consistent readings and read on! Here are my moisture meter readings over the past month or so for my indoor grow area. The best time to water is when the moisture readings flatten out. In my case it's about every five days: Here are the moisture readings for a similar pot located outdoors on my roof. It's trickier to water because the weather fluctuates a lot: I've been dealing with evil squirrels and other things lately so I haven't been able to water on the "best day" for the past month. Hopefully September will be better. =) If you do not have a digital meter, try recording your readings for a week or two in a spreadsheet or grower journal. You'll start to see a trend and get a feel for when the meter 'says' you should water and how thirsty the plant will look.