The only correct answer here is AS MUCH AS YOU CAN AFFORD
Don't buy it from Apple
While Logic and other softwares offer minimum req's, that only tells you a little bit.
If I were to follow the minimum system requirements with my Pro Tools rig, I wouldn't be able to play half of the sessions I do - or record into them without delay.
The RAM requirements are more dependent on the kind of track count and plug ins you will be using... and whether or not you are streaming video....
Some of my plugs are CPU intensive, some are RAM hogs, some are both.
All depends on what you are doing.
Also, running an SSD for the system drive, and also a separate SSD for your Audio drive, will make that joker scream.
I would suggest the Mac Pro tower... I hear SO MANY people recommending people laptops for intensive work, and it makes me cringe.
1st, no expansion slots (What if you want to run high end gear via PCIe?)
2nd, what about your dedicated audio drive? best solution is a second internal SSD, easiest on a tower
3rd, What if you are doing video work that requires more GPU than the laptops offer?
4th, What about the crazy load you put on em in huge sessions, and laptops don't dissipate heat as well as towers - you're baking that sucker - remember, RHOS compliance? Yeah, all the good stuff is gone now, even from the solder..... #1 point of failure I see in laptop MOBO's?????? --->>>> Lifted IC's due to solder letting loose from a couple years of baking in the laptop.
(Granted, I have a Late 2007 Santa Rosa MB, but I don't use it for intensive work much - occasional photoshop/illustrator, but that's it)
I have more reasons but these are the biggest
When I'm spending good money on a machine that I'm gonna beat the snot out of, I want the best chance of it being upgradable, and lasting a long time.
things to consider... everyone will have their own opinion, this is mine