Like others have said, Showa era dubs are usually fun and campy but the films are usually recut with varying results. For example, King Kong vs. Godzilla is probably better paced in the international cut but you miss out on Ifukube's god tier score.
Everything from the 80s to mid 2000s has some of the worst dubbing ever created. You'll probably get some "so bad it's good" enjoyment out of 'em but it gets real old, real fast for some people.
Shin Godzilla's dub was handled by Funimation so it's probably competent. Never seen the anime movies in English but there's no way those scripts translate well.
For what it's worth, the DVDs/BDs of Godzilla movies are usually bargain bin priced.
There is a torrent that pops up fairly often that has a bunch of older HD satellite broadcasts of everything from 1954 to Final Wars. Subs on them are either copies of the dub's script or fan subs that range from passable to straight up pulled out of their ass.