Which baseband version is the official one for the 4.3 I am confused between the official version mentioned in sam mobile and the one my country offers. They are both 4.3 but one is I9500XXUEMJ8 and the other is I9500XXUEMJ5. So which is the most updated one. I would guess the J8 but then I also need to think which one is the latest one for the S4 I9500 (Octo Core).

I have also seen several ROMS offering either one or the other. Getting me more confused in the process.
You didn't mention which country you are from.

As much as possible, use the version that is intended for your country and your network. I9500XXUEMJ8 and I9500XXUEMJ5 will most likely have the same features, but different modems (using a modem that is not intended for your country can give you a poor signal or cause other issues like heating).

I9500XXUEMJ8 is obviously more recent, based on its naming convention. If you're trying to flash the ROM from sammobile, you can still get the firmware that's not for your country. But you risk two things here (and possibly others): 


It may not have the region / language you want
It may not have the correct / suitable modem for your country.


In the second case, you can always flash a modem for your country (after you flash the ROM). But if you don't want to deal with possible issues (and headaches), better get the version that is intended for your country and your network.