Heater placement

Reznor has always been pretty good stuff. Don't get a so called "90 percent plus" or what is known as "condensing." These units require more service in general, are more fussy with heat exhangers, and in a garage / shop, freezing is a concern, as the heater always has some condensed water in it.

What you want is like the northern unit you linked to, or the Reznor pictured above. These are in what is loosely called the "80 percent" class. This came about as efficiency improved some years ago, the older units were backwards -ly known as "70 percent class" You cannot even buy a 70 class anymore.

Codes change, but there used to be a code for those type ceiling hung unit heaters that required 8 ft clearance from BOTTOM of heater to floor. You cannot get that in your shop, but I've seen many of these installed that ignored the code. There is also, generally, in any code, variance if you can talk an inspector into it. If you can, I'd probably hang it right above the bench in the left side of your photo.