So my question is: would it be better to get the higher version of windows so i can get more than 16 gb of ram?
If you buy te Home edition, you are limited to max 16GB of RAM. If you stay with our currnt PhotoShop version, you will be fine. If you decide to upgrade your PhotoShop version and/or start very large complex edititing, you may need more RAM.
cos otherwise i will drop down to 12
Stay with 16GB (2 x 8GB) using 2 slots (Dual Channel Mode) slot 1 and 3.
If you have 12GB (3 x 4GB) using slots 1 and 3 first (Dual Channel Mode) then adding the 3rd stick to slot 2 or 4 will force the RAM to run in Single Channel Mode resulting in poorer performance.
and it said that it does not come with a removable motherboard tray, I dont have that much knowledge in assemling the parts, so how is this important?
Not a deal breaker. Having a removable motherboard mounting plates is a bit of a luxury. Instead of installing the Mobo inside the case and then adding your parts, you can remove the plate from inside the case, fasten down the Mobo, install all the parts and then simple put the plate back inside the case.
Have you decided about the GPU?
If money is still a bit tight, have you considered buying all the parts, except the GPU, building the system, and buying the card in a couple of months.