about versions. i strong recommend 32GB instead of 16GB. The main reason is that this device cannot do swap of sdcard, then all your android titles will be installed on your device memory.
you can use solutions such mount folder or similar apps, but for easy and simple usage, is better to use the internal memory directly, without tweaking the device.
then, a microSD can contain roms, many roms for you can use within emlators.
summarizing: let the internal memory for native android stuff (apps, games, etc) and microSD for emulation stuff
about gamestreaming, shure you can play PS2 titles. All you need is a powerful PC for running the emulator, and a solution for streaming (Nvidia GTX for moonlight streming, or try other solutions such kainy, etc, etc...), a good wifi connection and you're done.
my suggestion. Currently, i?m getting better results with another PLC on my home. i use one PLC with Wifi for connecting my console, and get better video quality and response than connecting directly to my Wifi (that many times lack of strenght when i'm on some points on my home)
then, my suggestion is: buy a PLC for streaming stuff. Plug a PLC near your router (for connecting via ethernet cable), and the other plc can create a Wifi AP, that will be used for your device (GPD-XD) for connecting to your local net and stream from your gaming pc.