There's plenty of info in this forums about this.
I will tell you more.. Just by a quick look at the official ASUS specs page on the net, you could get almost all your answers answered.
YES it can be updated onto 32gigs ram divided in two 16gb ddr4 modules. As the asus specs web says;
https://www.asus.com/Notebooks/ROG-GL752VW/specifications/"DDR4 2133 MHz SDRAM, 2 x SO-DIMM socket for expansion up to 32 GB SDRAM"
YES you can put W7 onto the machine. You can put W98 if you want to. But keep in mind that ASUS will launch the drivers for W10 and the functions on previous OS is not guaranteed.
What SSD did you pursached? Most of the GL752 owners (included myself) have been installing SSD M.2 SATA AHCI mode successfully.
Yes, it has been released the info that this ASUS do only support SSD on the M.2 bay until 256gb. Anyway, the question about "500gb or 1tb ssd" simply relies on you, you and you are the only one who knows what kind of use is going to be made on the laptop. Myself, I think it's enough with a 128gb SSD for windows and basic apps. Only more if you really need the speed as you work with some proffesional apps.
Go for this one;
http://www.amazon.com/Samsung-850-EVO-250-MZ-N5E250BW/dp/B00TGIVZTW/ref=sr_1_1?ie=UTF8&qid=145246192...make sure to buy separately a screw for the installation. Not included on the ssd nor the laptop.
Last sentence you wrote; you have not been searching quite a lot, I'm sure here in the forums most people don't mind helping anyone, but it gets easier when the asker has just investigated at least 5 mins on the net.
Greetings