I had your exact case. I was running a 2010 27” with i5, 16Gb Ram, 256SSD + 3TB 3.5” HD, and the stock 1GB(512GB) GPU. It such a great system and I wanted to upgrade before even considering the price of a competent new iMac.
As of last month. I successfully upgraded the CPu for an i7 “4core 870” (38$ eBay), 32Gb Ram(88$ eBay 4x8gb), Samsung Evo 850 1TB SSD (99$ eBay) and a GTX 765M 2GB GPu (79$ eBay). I went full upgrade and paid way much less than just getting me a working i7 for the same year and i ended up with a super machine. Nit to mention Metal capability GPU that’s currently Mojave.
Altough i now run mojave, the Cinebench specs went down a few fps with the new gpu for some reason, so i would recommend a better GpU but only after someone else confirm they work as well as the 765M(ive seen some 880 8GB for around 250$). There are videos on YouTube and a lot of discussion around. I dont need the extra GPu since i don’t run any video making or anything rather parallels with Autocad.
There are a couple of details one should know, but i can explain details to anyone trying this. Most is explained in videos and forums.
I wish you success and that by now you had done it!