First game in the series now is
conveniently available on Steam.
It's already patched to latest version and contains all the campaigns released officially (including Panzer Lehr and Liberation of Belorussia).
The first game used an OpenGL-based engine and, since modern video card drivers sometimes lack full OGL support, it's better to try the demo before purchase to make sure it works OK on your PC. You can get the demo
here, for example.