I used a Taurus 2 speed fan. I created my own thermal control circuit with 2 temp switches in the heads and some relays (5 in all). Slow speed comes on at 185, Hi speed comes on at 200. I also have a manual override switch to kick it instantly to high speed. And it will also be enabled by the a/c compressor kicking on.
But all of these functions can be provided by some ready-made fan control circuits, with some being very simple for a single speed on/off at a specific temp setting, and others for dual fans and other features. I ended up spending about as much on components and wiring as purchasing a unit would've cost me.
I doubt you would be able to use your mechanical fan with the electric if your electric is a puller. If your fan is a pusher, you could probably do both.
Something like this is simple and to the point:
http://www.buyautotruckaccessories.c...FSoZQgodSQnAVw
Google "electric fan controller" and you find quite a few out there. Just be sure to search for some reviews and opinions, because I've read a number of complaints about durability from certain manufacturers.