Tkereacer619 is the man to talk to. He sold me my first cole parmer pump, and I have since bought two more off ebay. I run my calcium reactor off the one from him, and I run my auto water changer off a dual head ebay unit, and my sulfur denitrator off another single head ebay unit.
When you buy one off ebay, you should have in mind what level of noise is acceptable to you. The digital drive type is more expensive, but whisper quiet. That is the type I got from tkeracer. The variable speed console drives are very accurate as well, but a bit noisier. All my equipment is in an unfinished area of my basement, so noise is not an issue for me. But if your equipment and the pump will be located in an area you need quiet, you may want to pony up for a digital drive type, which is more expensive. I got my two variable speed console drives for about $200-$220 each with an Easy Load head on it.
I have my CP pumps pushing water through my reactors, but may switch to them pulling from what tkeracer posted above about them overpowering o-rings on reactors. Should not be too big a job to do that.
Another thing I would look at regarding these pumps is your pump head choice. Easy Load heads, wether Easy Load I, II, or III, are the way to go for ease of use. And,
I would also look for the Easy Load pump head models that accept the greatest variety of L/S tubing.
The wider the variety of L/S tubing the pump head installs, the more flexible the pump is in its uses, in case you ever repurpose it. A pump head that only accepts 3/8" ID tubing suitable to run an auto water changing system may not be very adjustable if you are only trying to dose 2 ml increments of amino acids should you use it for something else, whereas a pump head that accepts 1/16" ID to 3/8" tubing is.
If you buy a digital Console like the first picture I posted below, the tubing ID may not be as important, as you actually program the tubing you are using and the desired ml/minute rate, but I think the majority of users will buy the less expensive AC motor drives like I have pictures in my last two pictures.
Also, if at all possible, buy variable speed and reversible drive units like I have in the bottom two pictures below if you do not go with a digital drive. They make it a lot easier to adjust amount and flow rates.
