This will (probably unless I make a new discovery or something goes wrong) be the last update to this thread. I have been finally got this build to a point where I am comfortable to say that it works fine and is ok for daily use. The rest of this will be divided up into things that could be done better and information about how to resolve of the issues I was (and am) having. Notes:
If you are planning on going with an eGPU setup (these are not just M6700 specific and could help with related issues on other machines):
From what I've experienced AMD works better and i easier to set up, their drivers usually install with no modification and no problems (except for 19.1.0 caused BSOD but rolling back to 19.0.0 and then later updating to 19.1.1 fixed that), I've had issues with old quadros and error 43 and have heard about error 43 being an issue for new cards. If you are going Nvidia and are gettings error 43, @Ross Siggers pointed out apparently it can be fixed sometimes by simply modifying the device IDs in the driver (a process that you can find out how to do with a quick google search)
If you are using Expresscard (which I recommend over mPCIe because of convenience and no different performance (on EC 2.0 vs mPCIe 2.0) I have tried several combinations of things, here are most of them:
connecting the eGPU before boot allows it to work without error 12 BUT if it is disconnected the laptops internal display wont turn on.
connecting the eGPU anytime after the laptop is turned on results in error 12 or it just refuses to work.
however, turning on the laptop with the eGPU connected, entering BIOS password and DISCONNECTING the eGPU before Windows starts loading, then reconnecting it after the login screen appears allows both GPUs and the internal display to work fine including hot-swapping the GPU!
If you have awful GPU performance check task manager, if dwm.exe (Desktop Window Manager) is using a lot of GPU (more than the 3D application) the only solution I found for this was to either disable the laptop's built in GPU or set the internal display as 'Disconnect This Display' in display settings, this is true EVEN IF you aren't running the application on the internal display!
If you are experiencing random eGPU disconnects the only solution for me (as weird as it sounds) is to ground myself to a nearby PSU case using one of those anti-static bands, any combination of desktop PSU, DA-2, shielding, switches on the dock, whatever had no effect for me.
Laptops may seem similar, but REALLY CHECK components:
the M6600, M6700, and mostly the M6800 look VERY similar, however that does not mean parts are interchangeable! This is especially true for trim pieces that might change very slightly, the bottom panel on mine that I ordered was for the M6800 but it didn't fit without some modification, the keyboard surround from an M6600 or M6800 are NOT the same as the M6700 and do not stay on correctly. The GPU Heatsink from an M6600 is VERY slightly different from the M6700 and probably wont work. It's things like this that were annoying to deal with
Check items when they arrive:
This might seem obvious to most people but I'll point it out anyway, when a part arrives inspect it and MAKE sure it's what you ordered, I had the wrong screen show up and make me think there was an issue with my board, but it turns out my board was fine and just the LCD was wrong. I didn't have many issues with this, just the screen being wrong, the keyboard shroud being refunded because they didn't have it and the battery being DOA, all of which were resolved (except that damn keyboard shroud)
Things That Could Be Done Better:
I recommend if you have some extra cash or don't have a desktop GPU already and have an MXM slot, try and find a compatible GPU for your laptop, BUT if you want to save some money an eGPU is a more permanent solution that seems to work fine in most cases, the performance loss that people talk about isn't too bad (for me it was about 10-15% and I hardly noticed a difference day to day (though this could be because I have a way better CPU in this machine)
Old laptop HDDs are SLOW AS ****, my laptop has a RAID 0 of 2 750GB toshiba drives and they only get ~3MB/s random read and writes, make sure you get an SSD for your OS or else even a powerful machine will feel super slow!
120Hz displays on laptops are AWSOME, but keep in mind this isn't really a useful option if your laptop doesn't have a good GPU (NOT eGPU because the performance loss for me was awful, I went from 140-150fps on an external display to like 25fps on the internal display...)
Old 'Mobile Worksations' are big, heavy and have a TON of screws and fragile ribbon connectors, my Precision M6700 has ~70 screws holding it together total and ~40 that you need to remove to get to the internals which fortunately are only a few different sizes. There are also 8 connectors that are attached to JUST the top keyboard portion, be careful to not rip these when disassembling, fortunately the only cable that was damaged was the speaker one (a small 4 pin picoblade style connector) but it still works just the plastic is broken.
Some General Laptop Things:
Replace Thermal compound! It helps keep the laptop quieter, cooler and even faster in some cases, I recommend NT-H1 but any high end thermal compound is leaps and bounds better than the included paste.
Make sure you have compatible charger! If you are going Dell almost all dell chargers will be able to power their laptops but old workstations draw a LOT of power for laptops, mine will not run at full speed without a 210W charger. If you aren't going with dell make SURE to not just check the connector BUT CHECK FOR POLARITY AND VOLTAGE OUT! Plugging in a charger that fits into a laptop that is expecting a different voltage or polarity has a HIGH possibility of killing the laptop!
(This one is more just windows, less laptops specifically but they are moved more so I'm including it) Set a BIOS/Boot password, pretty much every computer has an option for this is the BIOS and it could help if your machine gets stolen, its common knowledge that Windows passwords are NOT secure, if you want to you can also use secure boot to protect data on the drives, my laptop even has an option to password protect specific drives in the BIOS. Power on passwords wont stop someone from just ripping parts out (including the drive with your data on it) but it might stop simple thieves from easily accessing your laptop and gives you some piece of mind.
This was a long one! If you have any questions (or notice something I did wrong), feel free to point it out/ask. I'll probably check back on this thread every so often, thanks!