What to Unlock First?

Item one, no doubt, should be that tablet. Trying to remember what parts to buy and running back and forth to the desk computer is a pain - not to mention an epic waste of time.

After that I would suggest unlocking the Path Test and the Compression Tester. Both of these are great as they can help diagnose parts that are either difficult or impossible to see without doing some disassembly to expose them. Try the Path Test first since it covers the most parts. Many of the ones it tells you about can be hidden from view. Follow that with the Compression Tester in order to get info on the Pistons without pulling the Oil Pan. After that the Electronic Tester and the the OBD Scanner will come in handy.

I put repair tools last because it's quite wasteful to try and repair without level 5 in Repair Tools, and that takes a good while to achieve. Actually, the very last thing to unlock is Fast Manual Examination. I keep forgetting it is last, because in my last game I completely forgot to unlock it for a very long time. Honestly, I think that in 100+ jobs and rebuilds I only used it a few times.