I base my 1/72 plastic figures on 5/8" washers and it works great. Make sure the washers adhere to magnets; that's not essential but might make life easier for storage and/or for re-basing for a variety of rules systems.
Of the three companies you mention, the only figures I've seen in person are Pegasus Hobbies. I have their Russian Summer and Winter infantry, which I think are excellent, and their other sets have received excellent reviews. Caesar is another company that produces fine plastic figures, in my opinion. I haven't seen the PSC figures in person, but they look to be proportioned more like metal figures. That may be a good or bad thing depending on your taste. Finally, I've found Revell figures to be high quality, as well. I have a set of Italeri (re-issued Esci) Russian infantry, which are "okay" but not great.
You should start looking at Plastic Soldier Review. It's an invaluable resource, especially when you're trying to figure out which boxes of figures to get for specific weapons or poses you need.
When you paint plastic miniatures, I recommend that you first coat them with watered down white glue. If you do a google search you'll find some articles and advice on this. I don't think it's essential -- the first figures I painted I didn't pre-coat and the paint hasn't come of or anything. However, the later figures that I did coat feel much sturdier and less "bendy" during play.