An apostrophe doesn’t really make sense there. Apostrophes typically stand for elided letters (e.g., “don’t”), and occasionally for a glottal stop, which “ع” is not (it’s a pharyngeal fricative).
There is also a tendency for beginning fantasy writers to come up with names involving lots of extra apostrophes and accents to make them look more fantastical—a phenomenon which I affectionally call the Dread Apostrophe. For that reason I’m not fond of apostrophes in names.
The standard way in English of indicating that two vowels are to be pronounced separately is with diaeresis, as in the word “naïve”. Saäd looks pretty good and would be my first choice—it’s also correct in French orthography. You’d be likely to run into issues of representation in computer systems, however—English-speaking operators would probably just type Saad.
If you must use punctuation, then a hyphen is also possible: Sa-ad. However, since the name is largely unknown to English speakers, they will probably pronounce it as “sad” or “sard” (non-rhotic) no matter how you spell or punctuate it.