@Stephen suggested I long-form this here after my recent repairs. I'm attaching the MX-500 Service Document to this post in case it goes missing, but it's invaluable for troubleshooting this thing!

I bought a "for parts" metcal mx-500 on ebay, and sure enough, it mostly didn't work.

On initial inspection:

1. Iron didn't get hot most of the time. Would randomly heat up after a while and work for a bit, then stop working.
2. Green light would sometimes come on, sometimes you'd just get the orange light

Opening the unit:

1. You need a security torx driver.
2. Only the 4 outer screws need to be removed to remove the front cover. The middle screw doesn't need to be removed to remove the front cover, but will need to be removed to remove the board from the back cover. DO NOT full remove this screw from the bar it's connected to inside - it's PIA to re-attach if you do :)
3. The IEC connect is connected to the board with spade connectors and can be VERY tight.

After opening the unit:

1. Immediately noticed the 180ohm resistor at R2 was completely fried, but somehow still read as a 180ohm resistor!
2. The drain at Q3 and Q4 were incredibly hot.
3. When the iron wouldn't heat, I was measuring 40v at the drain of Q3, and almost 60v at drain of Q4 (this is way too high!)
4. Referencing the service document (attached), my voltages at U4 or the capacitors further down the list, didn't match what was documented there.

Thanks for some great posts on EEV Blog (Post 1, Post 2), the attache Service Document and schematics, and the following list from one of those posts, I was able to narrow down the search a bit (thanks to EEV Blog user quadtech for this list):

In the case of my machine, I had 3 failed components... well 4 if you consider the toasted 180ohm R2:

1. R2 was fried
2. Q1 was bad, and read as a resistor in my component tester. This thing has a weird little heatsink hat with thermal paste... it's just friction fit and pulls off, but you'll want to keep it around to reinstall on the replacement :)
3. C4 was bad, and read as 1nf instead of 330nf... was also visibly swollen. I replaced it with a comparable TDK cap since the WIMAs are harder to find
4. C24 was bad and red at 65nf instead of 100nf. Visually, this one looked fine. I found exact replacements on digikey, but 2.5mm pitch on these style caps seems to be something that's not made anymore really, and this is a tight location.

I had a blast repairing this machine and look forward to others! Having a working mx-500 on the bench was helpful, but not necessary with the help of those other posts on the EEV blog! Hopefully someone will find this post helpful - and now I'm one step ahead when my daily Metcal decided to die :)

Just to add sample size here, just had another mx-500 on the bench. This one looked mint inside, but c4 was clearly melty and swollen. Machine was happy as a clam again after replacing it!

Love the update, thank you for sharing!