Where can I find information on MT and HE mechanics, like the job requirements at the different echelon levels, job responsibilities, etc.?

A good find is a Construction Mechanic Seabee who's going reserve. They're into heavy equipment, diesel, hydraulics, etc. You tend to find some E-5 transition mostly due to getting married, having kids, settling down, etc.

Face to face interviews is the way to go. Conducted by a Mechanic. I've discovered that everyone can write or has a marvelous resume thanks to ACAP and professional resume writers. I've hired Mechanics with 10 and 15 and 20 years in who turn out not to know a thing, (I refer to them as the E5 who hit the chair the day he was promoted and never picked up a wrench again) E4s who can spin a wrench like a maestro, SNCOs and CW3/4s who can't lead a person to water or interpret a simple maintenance report.
I'm glad you want to hire Military. There are a lot of experts out there. Just have to make sure you hire the right one. Read the resume, conduct interviews, hire a true mechanic who can also mentor and lead subordinates. There are lots out there.

I can't speak for the other services, but in the Army it's going to be the 91 series MOS' that you're looking for...

91A M1 Abrams Tank System Maintainer (formerly 63A)
91B Wheeled Vehicle Mechanic (formerly 63B)
91C Utilities Equipment Repairer (formerly 52C)
91D Power Generation Equipment Repairer (formerly 52D)
91E Allied Trades Specialist (formerly 91E and 91W)
91F Small Arms/Towed Artillery Repairer (formerly 45B)
91G Fire Control Repairer (formerly 45G)
91H Track Vehicle Repairer (formerly 63H)
91J Quartermaster and Chemical Equipment Repairer (formerly 63J)
91L Construction Equipment Repairer (formerly 62B)
91M Bradley Fighting Vehicle System Maintainer (formerly 63T)
91P Self Propelled Artillery Systems Maintainer (formerly 63D)
91S Stryker Systems Maintainer
91X Maintenance Supervisor (formerly 63X)
91Z Senior Maintenance Supervisor (formerly 63Z)


91L probably focuses most on the specific skill set you're looking for. All of the mechanics that I've worked with have been up to their elbows in it from E-1 - E-5. I've seen E-1 - E-6 manning tool rooms. E-5 - E-6 is where the supervisory roles usually start. How the duties are delegated to the ranks usually depends on who's available and what's available.