Processor choice? / Home Studio? Running 80 tracks on Logic Pro X

I'm not saying you have to buy new, but I'm saying that you have outlined a need for both a quite powerful machine that can use lots of RAM, plus a need for lots of fast external storage, plus an (unstated) need for a backup system to protect that investment. That's not going to be done on the cheap is what I'm suggesting. I've put in a message to one of my LA soundtrack-guy friends and we'll see what he suggests.

Wow, thanks for that. I appreciate your help!

You need to be asking those using the software/hardware you intend using.
Those that have responded here thus far have all admitted we are not doing what you want with our machines & are only making our best guess.
We could be all wet behind the ears.

You really need to go find and ask this of users doing what you want to do.
Look up a Logic Pro specific forum.
Here is the # 1 hit from my google search.
Searching yourself, could possibly find a forum based there in the UK.

nate . . . you really need to RTM that comes with Logic Pro X to see how many cores it can utilize. The number of cores usable is determined by the software. No use in buying a 12 core processor if Logic Pro will only use 2,4, or 6 cores.

Nathan: I heard back from my composer buddy at last (he was sick with a cold). He basically concurred with my analysis that you budget was a little too low for what you want to accomplish. He uses the last of the "tower" Mac Pros from 2012 loaded with RAM and said he can handle the track number you suggested on his machine, but he tends to separate the composing/compiling portion and the mixing portion into two parts so as to improve performance. So "our" mutual suggestion is to find a good 2012 Mac Pro or later (my pal is about to get one himself), add tons of RAM, and enjoy.

Wow, thanks for getting back to me. I had a look on eBay and for a similar price I can grab the new Mac Pro 2015, and this should have no problem running everything? hopefully?

As I say, that's the one he's getting for himself. It's a Pro machine totally designed for running Apple Pro apps, so ...