Email Brian Baskin at DC controllers and ask him your questions - he can tell you what hardware you'll need. He has WAY more experience than me in this arena - he knows his stuff. I'm gonna guess he's gonna say the 35A unit is more than enough, and you'll probably wire it to the high speed wiring so it has the potential to run the fan full out; if your car doesn't require all the fan speed to keep it cool, it simply won't run it at top speed. Yes, it will allow you to totally do away with your relays, and that primitive manual switch. You'll be able to select a control temp and forget about it.