Not sure where are you. The Dell 30x0/70x0/90x0 MT is relatively cheap nowadays on used pc market. I think the barebone could go as low as $50 with power supply.  Rather than buying parts to fit in your DT model, better get a MT model and re-use some of your DT parts (CPU, memory, Storage etc.).
I want EVGA NVIDIA GeForce GTX installed if possible.   I understand that I need a 400W power supply upgrade to make this work.  If I get something like: [URL]   Do I need to change anything else on the motherboard?  Will this work?
Optiplex 3010 GPU upgrade to EVGA NVIDIA GeForce GTX 1060 6GB SC Video Graphics 
GPU.