The Pex tubing is actually not very expensive, you can install it and just not use it. Install on 12" centers, pressurize to 20lbs and pour the slab. I would put down a reflective bubble insulation at minimum below it, that can also act as a moisture barrier for the bottom of the slab. I love my garage floor heat, it however doesn't heat up very fast so I want to install a forced air heater. I had a small 1500 watt electric heater that I put near me when I went out for short work session one winter and it worked ok. I keep my garage between 48 and 55 as the base temp then if I have a weekend project I kick it up to 65 on Friday morning and hold it there all weekend then let it cool off starting sunday night. It usually takes until Tuesday morning to get back down to the setpoint.
Down the road if you come into a low cost boiler or heating system that you can hookup to the tubing then you get floor heat, it's awesome heat for working on things, especially when your under a vehicle.
I only have R-19 batt in my garage walls but do wish I had done more, that said most of your heat loss will be from the ceiling. i would do R-21 batts in the walls and R-50+ in the ceiling if it was me. If you can do more wall insulation do it but the ceiling/roof is more important.
Big garage doors are expensive and almost not needed, I would look at a simple 9'X9' door and then a 12'X12' door. I would love to have a shop that I could just back a trailer with a rig on it into without much issue. 90% of the work you do though the 9'X9' door will sufice. I can put both my snowmobile trailer and buggy trailer in my 9' wide door but only have about 1/2" on either side of clearance.
One thing I notice in the winter is that most of what I move into and around my garage would fit through a 4' to 5' wide opening. You might think about installing a pair of doors for the main walk door. A pair of 3'-0 Metal doors without a center astragal would allow you to have a near 6' wide 6'-8" tall opening to bring things like snowblowers, lawn tractors etc. in and out without opening the big doors. Not sure if that's worth it or not?
Plan on a 5" slab with 12" thick pads for a lift, you can even block out the pad areas and pour them "seperate" so you know where they are and can avoid putting your Pex in those areas.
All I can think of for now,