Which options for new 27" iMac re: lightroom?

Please advise on a new 27" IMac purchase, specifically for 1.lightroom and 2.photoshop (in order of priority).

Funds are not unlimited, so best bang for the buck and hoping to future proof for 3 to 4 years of usage.

Specs:

3.2GHz Quad-core Intel Core i5, Turbo Boost up to 3.6GHz or

3.4GHz Quad-core Intel Core i7, Turbo Boost up to 3.9GHz?

8GB 1600MHz DDR3 SDRAM - 2x4GB - (Ram will naturally be added to later)
1TB Fusion Drive (probably the best option?)

NVIDIA GeForce GTX 675MX 1GB GDDR5 or​
NVIDIA GeForce GTX 680MX 2GB GDDR5?

Thanks in advance.

--
Rudi - freelancer
'Never mind the quality, feel the width'