I would do what PHJ426 said and get relays with additional logic (more open and closed contacts) to them to prevent having the low and high speed on at the same time. Everything else looks like it would work. Really, getting a relay with additional contacts should be fairly easy. Look in a Grainger catalog or just go to your nearest Grainger or electronic store and ask for a 12VDC activated coil with DPDT contacts. They will hook you up. Then wire your switches into the relays so that if the low is on the High never comes on or visa versa.

-Don