basically, most of us here utilise sump like a storeroom...to store all the stuffs like skimmer, FR, refugium and watever not...it is also used as a mechanical filteration where u put the sponge to take away huge particle dirt...other than that it creates a bigger water reservior which means your whole tank setup will be more tolerant to chemical changes...
whether 2 or 3 or 4 compartments depends largely on wat do u want to put in it...in my opinion, make it as large as possible with less compartments so that most of the things can be dumped inside...and it creates a larger water reservoir as well...
as for kind of filteration media, i recommend only sponge...no bio ball as it will become a nitrate factory as discussed before...but it all depends on your preference, some swear that bio balls are the best but other say NO to bio balls...i follow suit and utilise the most prefered method...without bio ball...
try to have more LRs and a DSB in your main tank and leave your sump as empty as possible so that u can put more equipments in it...as u mentioned,u want to have a reef tank, so bio ball should not be used..the reason being bio balls tend to break down the DOC into ammonia into nitrite and nitrate very very fast, thus the nitrate factory theory...but DOC is also toxic to most fishes and corals so to curb this, a mighty skimmer should be used...