You will find that water temp variations are not trivial to deal with. To get 2.5-3GPM through a flowmeter, radiator, waterblocks, filters (you SHOULD use one to protect the flowmeter from teflon bits and other junk), and valves and crosses and plumbing will take a powerful pump. Your powerful pump will make the water temps increase as you throttle it. Even with a chiller you'll probably have to adjust the set point downwards to keep temperatures constant over the whole flow range.
I have had some luck using a rheostat and fans on a radiator. I check out equilibrium water temperature at the lowest flow rate first with fans wide open. I then back the fans off a bit so that I have a bit of margin for error and use the water temp at that point (probably 80% power or so?) as my "set temp" for the water. As I increase flow rates, I can lower fan speed to arrive at the same final water temp. It takes a ton of fiddling and is not user friendly at all. I have my testing equipment in the basement room on a table that doesnt face any external walls, and I have a couple of house fans blowing onto the testing area at all times.
It is basically the opposite of "automated" but it can work with practice. I bought a PID controller thinking I could make it PWM the fans using a type t thermistor in the water as it's input (compare it to set temp and PWM as needed) but I haven't gotten it to work yet.
If you are serious about getting a chiller then search for "recirculating water bath" and get one with heating and cooling capabilities. You will be no better off than I am with the rheostat unless you get a digital one with good temp regulation. The one I have at work for reaction kinetics is +/- 0.1C from 5 to 60C and was a vwr brand. Haake and Lauda make good units but were more expensive. I think I paid $2000CAD for my unit but it was listed for much much more. I added it onto a sizable order to get a break
