Here's what I suggest. With CAD, I have a friend who just built a desktop mainly for that purpose. We used Nvidia and Intel build, but AMD works too.
For GPU, Quadro is the way to go, get the best on your budget, but if you want a cheaper option, AMD FireGL and FirePro. The issue is gaming. They do work with games, but you will probably have to compromise to either get better gaming, or bedder modeling and drafting.
For CPU, get any standard I7 for easier board compatibility. Xeon might be out of budget. We got a 4790(~$250-300 on sale, sometimes without a stock heatsink) from Intel's arsenal. The GPU is really the biggest decision though.
Ram, 32 max probably. DDr3 and ddr4 aren't that much different, but shoot for 16-32GB either way. We just went with ddr4 because we could upgrade to 64Gb later.
I'd also recommend SSD over HDD anyday. Personally purchased https://www.amazon.com/Samsung-850-PRO-2-5-Inch-MZ-7KE512BW/dp/B00LF10KTO/ref=sr_1_1?ie=UTF8&qid=1501197103&sr=8-1&keywords=samsung+pro+ssd and it works great. It's not in RAID, and it's the only drive, so any games and software as well as OS are all on that drive.
Those are really the biggest things. Since his isn't for gaming, we didn't worry about that aspect, but in your case, I'm faily certain you will need to compromise on CAD or gaming, because the GPU is specialized for the process.
At your low budget, you'd need a pretty expensive workstation card to game with the performance you received before.
Best of luck.