New computer contemplation - specs and dilemmas

My iMac is about three years old. It’s kicking along fine, as long as I don’t do anything too hardcore in the 3d department. We’re having slight issues there, as in Blender starts lagging and acting strangely (like failing to realise that I’ve “released” when the pen is off the tablet) when it has to deal with over 2M polys.

This is the machine I want to get:

Mac Pro

• 2.7GHz 12-core with 30MB of L3 cache
• 64GB (4 x 16GB) of 1866MHz DDR3 ECC
• 256GB PCIe-based flash storage
• 2x AMD FirePro D700 GPUs with 6GB of GDDR5 VRAM
• Apple Thunderbolt Display (27-inch)
• 3 year warranty

Unfortunately I don’t have $14 197 kicking around (and if I did I wouldn’t after I’d dropped it onto the mortgage). It goes down to $12 097 if I go down to the 3.0GHz 8-core with 25MB of L3 cache and $9 997 if I settle for the 3.5GHz 6-core with 12MB of L3 cache. Additionally I’d have to get some extra drives for storage because I opted for the smaller built in drive with the intention of expanding externally. Ideally I’d like one of the Pegasus RAIDS that you can get bundled with it but unfortunately I’m not yet a proper studio with a budget for these things (as is I’m lucky to be in a position where I can start scraping together money for the next machine as soon as I’ve bought one as it takes about that long to save up, but sometimes other things like unexpectedly large vet bills or planning on buying another house and moving into it can interfere with that).