looking for advice for which system for fcpx

Hello.
I have been editing using a macbook pro (13 inch 2011 with ssd, 16 gigs ram external monitor) to edit with cs6 for some time now.

I am going to order a new Imac next month. Mainly to future proof. I am a filmmaker and photographer, and want to be able to edit HD and 4k in FCPX. I will also be using CS6 for existing projects until I learn FCPX

Money of course is limited. I am considering a refurbished 27 inch imac retina 2015 with 4 gig video card and 2 tb fusion drive. To that I intend to add two 8 gig ram chips or if money permits two 16 gig chips.
additionally I plan to order two 240 gig usb 3 ssd drives and raid them for a scratch disc/working disc.

I have a nas back up drive and several fw800 drives for storage and back up. the plan is to use the ssd raid while working on a project then transfer it to the other drives for storage and back up.

so the questions..
is the 4 gig video card worth the extra money or not?
is a stripped raid ssd set up a good idea for working disk?
How much ram is sufficient? keep the stock 2x4; and add 2x8 or spend more and go 2x16?
any other advice or tips please?

my understanding is my old aftereffects cs6 won't even use the higher end graphics card on the mac, is that correct?