Start with a larger building, both width, length, and ceiling heights, then sit it where you add onto three sides with leans if needed later on. I love my service pit dearly, wish I had two in fact, mine is 50 feet long and its still not enough at times, we put several things over it at once end to end and have need for more at times.
If your going with ag repairs as well, you need a wider door or you can't get much in ag related, combines with heads won't even begin to fit through a door that size, not to mention have a place to park or unfold anything once inside, but I have no idea what kind of ag equipment your working on either.
My shop ceiling is 20 feet tall, people thought I was nuts for going so tall and now it has dings in it from hitting it with the crane, you need to figure out height and then the height to clear what your taking off the machine as well as the crane your using to do it with, I'd never go less than 22 feet again, maybe even higher if I were to use a steel truss open rafter design the next time around if I were to do it again, I'd never build with a wood rafter for a shop just for that reason.
I'd do a lean for everything short needed, instead of a loft in one corner or end, I hate to climb stairs and for the cost, its cheaper to add a lean rather than put a loft in a tall building.
I used blown in fiberglass in my wall cavities, and spray foam over cement block walls then fiberglass bats over that with tin on the inside. In my opinion you can't have too much insulation or r value for that matter. I don't like cement block at all, cement is porous and air blows into it and through it, not a lot but if I were do it again, I'd never use cement blocks at all anywhere, it would save a lot of grief insulating it later on.
I put infloor heat, I love it, but in frigid temps long term this past winter mine and nobody I know, the infloor kept up for extended cold periods, plan on two heat sources right from the start. I also did my in floor heat differently than most do, I made mine zone orientated not brining every line back to one location, that way if one line leaks, I can jack hammer out that zone and redo it, we just used several manifolds to do it with instead of one central manifold, something I really like. A cement guy told me to do it this way, he's seen problems over time with one zone leaking and instead of replacing the cement in the whole building, you just end up shutting down one zone. By separating them, I can just replace one whole section of concrete and piping.
I'd go no less than 7 inches thick on concrete with rebar.
I put pvc lines under the insulation and below the tile layer under the floor to pipe water lines, air lines, electrical and even pipe out waste oil without any of that stuff on the walls, I'd do again in a heartbeat, I love that idea. We also did that to put vents out of the service pit, which work great to suck stale air out of blow in during the summer to cool the pit area.
I put a drain in the bottom of my service pit with a sump pump in it, it works great, kicks in every 15 minutes to pump out ground water 24/7 and we put a lot of drain tile under the pit, around it and under all the concrete flooring, piping it all into the service pit, there are some in my area that didn't do this, now they can't use the pit, it constantly has water in it and its moldy down in the pit area.
I wired my pit, have air in it, lights built in and forced ventilation, all are a must, along with ladders throughout.
Bathroom, wash area, office/break room is a must, we love ours.
Even if you don't go with in floor heat, still put down the foam board, it helps to eliminate the condensation in the summer, which is a must as far as I'm concerned.
I don't like overhead rolling doors, roll up doors, but I do like my one piece hydraulic doors, would never go with anything else for a shop door. Just a few ideas I like or would do again, not sure any of them help or not.
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