Is it possible to mod both PC versions of GTA 4?
Solution 1:
There is no difference between the two versions, at least not when it comes to modding or actually getting it to run.
The Steam version of the game does not use Steam Workshop so all mods are handled through other sites. Steam will not make it any easier to mod.
Regarding getting the game to run, the error is almost certainly not caused by the Steam version. If it was, it'd probably affect other Steam games. Buying the retail version will probably not magically fix it.