Hey Rudy I was looking for a set of figures to go with my Korean War M40 SPG and couldn't find any either.
Masterbox or MiniArt, I forget which makes this set, but it's the early/middle Second World War uniform which is incorrect for the Korean War.
Dragon does make these three sets which are correct with the late Second World War uniform. US troops in Korea as well as the South Korean Army were given these.
I was going to kitbash the upper set with the lower ones to try to get what I want. Maybe you could do something like this? Hope it helps some. The Advance to the Rhine set is really useful, I've got about three boxes now and have used one for kitbashing different projects.
As to groundwork I've been using Durham's water putty. I find it stronger than plaster and doesn't chip. But if you're looking to put footprints or tire tracks etc in it do it while it's wet. They advertise it as setting up as rock hard and they ain't kidding!