Ranger:
id tech 1
http://en.wikipedia.org/wiki/Id_Tech_1
was the doom engine, tech 2 encompasses the quake1/qw/q2 source code base. They do have some differences but not a complete rewrite as Doom -> Quake -> Quake3 was.
If you want to make a mod that will have longevity use the Source engine. Otherwise, you will end up recoding 10 years worth of engine-side features only to find yourself ten years behind the then present tech.
If you truely want to just LEARN, grab a C/C++ book.
If your heart is totally set on Quake2 theme/media tinker with modding the Qfusion source.
QW -> Q2:clone? I think DarkPlaces and FTE already have that covered.