I just purchased a 27" iMac last month, primarily for photo editing with Aperture & Nik plug-ins. I can tell you that it works really, really well at this task and is an absolute joy to use.
Given I've been waiting for a while to purchase and had saved up, I opted for the i7 processor, 1TB fusion drive and the 680MX GPU. I ordered with the stock 8GB ram and upgraded it to 32GB. However, had I been on more of a budget, here's how I would have approached the purchase:
1) The i7 and 680 are complete overkill for photo editing IMHO (not so for video editing however). Even the stock i5 is probably more than enough horsepower for Aperture (or any other editing suite). Others have said the same in this thread;
2) Adding 32GB RAM has proven to be simply nuts, even though it was much cheaper than buying it from Apple. A quick glance at the Activity Monitor shows how most of this RAM remains idle ;-) That said, I would recommend adding more beyond 8GB. Many folks here have bought a 2x 8GB pack from Crucial and taken their systems up to 24GB, which is plenty;
3) For photo storage, iTunes libraries, I think that a 7,200rpm stock disk is plenty fast enough. I would probably order the 3GB drive as it will retain its performance better than a 1TB drive as it fills up plus it's difficult to upgrade internal components (other than RAM) after purchase;
4) Where the Fusion drive excels is in providing lightning performance for system files, apps, scratch disks, etc. You will really notice this in terms of iMac startup times, app launching times, app performance, etc. The good news is that you can start with a stock 1GB/3GB spinning media and upgrade to running your system disk on SSD after the fact. The iMac's thunderbolt port makes all of this possible. See this thread
https://forums.macrumors.com/threads/1489710/ for information on how this is done.
5) You can also use Thunderbolt to add high performance external storage as your photo library grows. There's plenty of storage vendors selling RAID enclosures with wicked fast performance.
Good luck with your purchase. One more thing to consider. If you opt for a stock entry level 27" machine, not only will you save lots of unnecessary $$$ but you can walk out of the local Apple store with one under your arm on the same day. I had to wait 4 weeks for my build-to-order system!
Nikko