Help me buy a new iMac retina with options

Hi everyone!

I am looking to purchase a new iMac 27" retina display model. My previous workstation has been a 2007 Mac Pro that I still dearly love but it's aging and cannot keep up with the latest technology.

One of my areas of uncertainty is between a mid 2015 model or a late 2014 model. If I can get more bang for my buck with the 2014 model and not sacrifice on anything, I'd be game for that. Any suggestions here?

Also, I ave a 24" apple cinema display that I would love to be able use as a dual monitor and I was wondering if the 2 GB GPU would suffice or whether the 4 GB GPU would be a better option in the long run. I've heard varying issues about the faster card running hotter. I mostly do photography/Photoshop/Lightroom and occasional video editing with iMovie or Final Cut Pro.

I'm fine purchasing the stock 8 GB memory and upgrading to a larger capacity. I cannot make up my mind over 1 TB fusion or SSD (512 GB at minimum). I have 3 hard drives from the Mac Pro that I would be bringing with me and getting some sort of enclosure or swappable hard drive dock. If I go the fusion route, is it replaceable in the future with pure SSD? (I have a friend who is a certified apple tech and could help me install this in the future). I want to spend no more than $2700 on this system.

So, in closing:

Late 2014 or Mid 2014 model

2 or 4 GB GPU

1 TB fusion or 512 SSD

Thanks for helping me think this through.