I was kind of half/half on Joseph/Caesar but in the end, not that all much.
I was kind of astounded that I really didn't ship it because they're pretty adorable together BUT I JUST COULDN'T DO IT.

I can't tell if Part IV is hard to read because of badtranslation or maybe its not that interesting???
It's a bit different from the rest of the parts, since it's a slice-of-life story with a serial killer as the main villain, but the bad translation brings it down A LOT. It has been redone with better translations, more than halfway though: http://www.batoto.net/read/_/123001/jojos-bizarre-adventure-part-4-diamond-is-unbreakable_v29_ch266_by_4chan or downloads here: http://donebetterelsewhere.blogspot.com/

Vento Aureo unfortunately has the same translation problems and only two volumes have been redone so far: http://www.mediafire.com/jojoproject#undefined

If you skip 4 and go to 5, the only real spoilers are that two characters from part 4 didn't die (they're at the beginning of the story). The rest of the story is fine to read on its own.

Skipping to Stone Ocean just spoils like one character not dying from part 4 (and mentions another one later) but the rest is fine on its own.

AND STEEL BALL RUN DOESN'T SPOIL ANYTHING AND CAN BE READ AT ANY TIME (THOUGH IT HAS MORE OF AN AMUSING PUNCH IF YOU READ PARTS 1-3 FIRST). 8D

I skipped 4 and 5, at first, because of the terrible translation problems and I think it's fine to read them last or later or whatever.