Dave,
I use one of the various pH "pens" that are on the market. I believe mine was from Cole Parmer and cost about $60. As for temperature, a good quality thermometer can be had for less than $10. A multipurpose digital meter is nice for these functions, but not necessary.
An oxygen meter is beyond what most pondowners need, but a YSI 55 meter is hard to beat. At $600, it's a bit pricey. Aquatic Ecosystems 1-877-347-4788 has some cheaper O2 meters, one being $225, but I don't know how good it is.
A standard Secchi disk works great for testing water clarity for fertilizing.
If you need the exact specs, I can get them for you.
