The surplus parkas are pretty tough and can with stand fairly serious abuse. As you mentioned, they are not warm as a stand alone garment, but layered they are a pretty decent outer layer. They are not good in the rain. They will shed water for a little while but eventually start seeping through slowly, especially if its more than a light drizzle. They are more designed for snow and general cold dampness, but not as a serious rain coat. This is why the military is issued a separate rain jacket shell.
As a travel garment, they do not pack very well and will take up a lot of space.
Honestly, if you want a garment that can withstand snow, rain, and general wet-ness, I'd spring for a good quality tough outdoor rain jacket shell (and maybe some pants too) and just layer up or down underneath for the different seasons. When it comes to outdoor attire, the civilian market is way better than the military stuff. The military stuff works, but there is definitely better stuff out there. The outer shell style will pack a lot better and not take as much space/weight.
Just my two cents after having used both military and civilian stuff in both tactical and civilian outdoor environments in a variety of different climates.