The problem is the dissonance between your wants and needs. You did not mention earlier that you already had a PSU. If I had known that, it would have changed my thinking. (although I would not recommend that PSU for this system)
When you talk about the limits of expansion for the mITX, exactly what expansion do you have in mind? The board will handle WiFi, and 16Gb of RAM and any GPU your PSU will drive. It has a decent number of SATA connectors. We have built several mITX systems, and a mATX system at school.
We are not suggesting the GTX750ti because it is better, cheaper, or more cost effective than the excellent 950, we are suggesting it because it is the best GPU that will fit in the slim 100mm or so case that you want to use.
This is a new system, so I am reluctant to suggest a CPU like the X4 860k, which for all it's fine qualities, is still a lower-mid-range CPU, but the top of the range for your motherboard. There is nowhere further to go, and I don't see that changing.
To get the full-size card you want to use into a 100mm case requires a case with a riser, that's pure design, or you can mod. it yourself. This person rotated a GPU using a riser cable.
http://www.overclock.net/t/1163799/nzxt-gamma-90-mod So, if you get the Silverstone ML03 and a mATX board, and a flexible riser cable, you should be able to mod. your case 'easily' (easy for me to say) to fit the card, cool it, and use it.
Overclocking the G3258 is very easy and safe, my Middle School (Intermediate) students can do it and the stock cooler is very stout.
From what you have said SOMETHING has to give. There is no solution for exactly what you want.
If you lived in the US or had access to the Azza Z case, then this would be the closes to your needs, except for the mITX motherboard.
PCPartPicker part list /
Price breakdown by merchant
CPU: AMD Athlon X4 860K 3.7GHz Quad-Core Processor ($68.89 @ OutletPC)
Motherboard: Gigabyte GA-F2A88XN-WIFI Mini ITX FM2+ Motherboard ($81.00 @ SuperBiiz)
Memory: Kingston Savage 8GB (1 x 8GB) DDR3-1600 Memory ($41.85 @ Newegg)
Storage: Kingston Savage 240GB 2.5" Solid State Drive ($99.99 @ Adorama)
Video Card: EVGA GeForce GTX 950 2GB Superclocked Video Card ($149.99 @ SuperBiiz)
Case: Azza Z Mini ITX Tower Case ($45.98 @ Newegg)
Power Supply: Corsair Builder 500W 80+ Bronze Certified ATX Power Supply ($35.99 @ Newegg)
Total: $523.69
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-09-04 11:58 EDT-0400