Unfortunately for you, you've just opened up a huge can of worms (for yourself).
Fortunately it's a magical adventure!
Here is the biggest tomb of knowledge that exists on the topic anywhere:
I actually own a Fostex T20RP MKII modded by the author of this article, and I can testify to these mods - they sound absolutely brilliant. I can only imagine how good a T50RP must sound with these mods.
I've never heard of two versions of the T50RP... maybe you've read about the original and the MKII which is the current production model. I wouldn't worry too much about that, and just get the MKII and enjoy the modding.
Whichever you get you'll most probably want to mod it.
Edited by GREQ - 10/18/13 at 1:12am