G'day Michal, and welcome to the fourm.
What experience do you have with DBMM? Not much?
If you have little experience with DBMM, my first suggestion would be to use post-280 Seleucids if you have any choice about it. The pre-280 list gives you a lot more elephants, but at the cost of including compulsory Ax (O). This makes no difference in DBA, but is a more serious issue in DBMM. In the post-280 list, the Ax (O) are replaced by compulsory Ax (S). In DBMM they cost only 1 AP more, but they are 1 Morale Equivalent each, where Ax (O) are only 0.5 ME each. DBMM also makes (S) troops much more powerful than (O), and Ax (S) have a Quick Kill against Spear. Elephants are a powerful weapon, but they are hard to use well. Altogether, I think this makes the 280-205 Seleucids more useful than the 320-280 Seleucids. Of course, none of this stops you from having 320-280 Seleucids for your DBA army - all of the elements you'd need in that would already be in your DBMM army.
Having said that, when putting together a Seleucid army, I recommend a very simple structure, with few different troop types. The Seleucid list provides so many useful troop types that it's tempting to use many or all of them. In my experience, this is a mistake. Concentrate on Pike, Auxilia and Psiloi for the centre, and a mix of mounted troops (Knights, Light Horse, Elephants and Cavalry) for the flanks. Don't bother with Artillery, Expendables, Warband, Hordes, Bow or Camels. This organisation then suggests a simple deployment plan - infantry in the centre and mounted on the flanks. And this, in turn, suggests PIP dice assignment for DBMM - lowest PIP dice in the centre, and higher PIP dice on the flanks.
Good luck with your army.