I'm like to reuse the following:-
Intel Core i5 760 - Gigabyte GA-H55M-UD2H microATX Motherboard - Corsair Dominator 4GB (2x2GB) DDR3 PC3-12800C9 1600MHz Dual Channel Kit
I also have a full size Asus high end sound card that I'd ideally like to use, but guess I could use a 90-degree adapter to get it to fit into a small form factor case maybe?
Hard drive will be a 2.5" SSD
Slim optical drive is just fine
Since you are reusing a lot of hardware, i think the GD06 you initially picked its a good one, given that its not oriented for quiet operation, undervolting the included fans or going with aftermarket fans (like Scythe Gentle Typhoon D1225C12B1AP-11
or GlideStream SY1225HB12SL
), and using 2.5 ssd it should end up quiet.
You have cover the hdd noise by going with an ssd, and probably undervolting the fans or going aftermarket will make it very quiet, one other source of noise is the PSU, i dont see you listed it on the reusing hardware, so not sure if you have one already or not, if you are looking for one, Corsair CP-9020066-UK RM Series RM450 80 Plus Gold 450W ATX/EPS Fully Modular Power Supply Unit
I didnt see a CPU cooler on the list, not sure if you are going stock, but one that its not that expensive that might be good in your case is Scythe Big Shuriken 2 Rev.B
, cross check if it will fit with the choice of optical drive.
I didnt see if you have a dedicated GPU, from what i remeber the i5 750 didnt have a built in gpu.