let me answer few questions for You, to kick start conversation with some recommendations.
Your 750m gives around 12-13 in terms of OctaneBench & then in Your silver Mac You should have a possibility to add pair of 980Tis - & these would give You around 250 in OB, so the work that is done in one one hour, on Your Frankenshtein would take like ~3 minutes =)
don't buy 1080s/TitanX (or any other pascal cards yet), but anything from Maxwell line like 980Ti or TitanX would work great =) also, You would probably need external power supply
- so with soem small mods You might be able to keep Your mac & enjoy working on it!
Pascal based GPUs (1070/1080s) are very efficient & even 1070 are close to performance of 980Ti if You got good cooling (& that matters a lot), so if You're going to choose big a** laptop, do not look too much to outside & think about things like proper ventilation (obviously, good screen, etc.)
So what's next? EGPUs is a pain in the back..- those are teased for years, but honestly, there isn't solution on the market that just works without any issues & would be compatible with wide range of devices.. if You need somewhat portable PC-laptop, look at razer + razer core
(not the cheapest choice, but..)
Screen wise.. all those beefy laptops are mostly oriented to gamers, meaning You would usualy get fast but far from accurate screens & then even PRO labeled screens are with cosmic deltaEs (in a range of gimmicky touch bar, but that's another theme on it's own).
Alternatively You can get a bit better laptop & look for something like Thunderbolt 3 via USB type C unit from companies like Akitio.. however compatibility is not guaranteed & You need to test it on a case by case.. (seems to work with some laptops & things like Skultrail NUC, but then no go with Apple's new laptops..)
Dual GPU laptop will perform twice as fast compared to single GPU laptop (with the same card =)
The best expansion is none (if You wish to avoid problems). Getting powerful enough laptop to make some & having somewhat portable powerful render render node that could be used as standalone computer would be Your best bet.
External GPUs are not limited connection type, but rather capabilities of Your computer..- You can find cases that would hold multiple GPUs.. but the problem might be to recognize those cards or even boot with that device plugged into laptop. Another issue is stability under full load.
That's why I would personally look at something like small enough laptop with inbuilt discrete GPU like 1070 & integrated GPU (to power screen while more powerful card would render) & then something like proper small computer, based on smallish MATX motherboard with 2-3 GPUs so You can easily pack for traveling if needed.
Feel free to ask any questions. I'm pretty sure guys on this friendly forums will be happy to give You some advice & different perspective as well. We all have slightly different needs & preferences, not to mention taste & the way we look at things, so put Your needs, try to learn as much as You can about different options & eventually You should be able to make educated decision.
Good luck! (& don't be afraid to ask if You have any more questions =)