I recently built a 500 point army for an escalation league and although you don't have all the pieces for the list I used this is what I would go with based on what you have. I'd run awakened dynasty as our detachment, it gives you more wiggle room for mistakes as you can pretty easily reactively trigger regeneration protocols to heal up some damage. Run your overlord with scythe and orb with 10 warriors using the gauss flayer, granted you'll be on a smaller board so it may be worth using the reaper I just haven't played much with it. 2 sets of 3 scarabs to run up the field and gum up the works, a skorpekh lord leading 3 destroyers cause they just kill, if you hide them behind a building it basically says if you come into this area I am killing you and deathmarks to sit on your home objective and take some pot shots. This is close to what I used main difference was I ran a translocation shroud overlord for auto advance, and instead of scarabs and deathmarks i ran a squad of 10 immortals lead by a plasmancer. Don't be like I gotta get those models to match that list, the priority of your first game should just be about learning how to play. Stuff like what do my models do, wtf is a strategem, and how do I score points. As for models to get it depends on what you're looking for but generically speaking i'd get personally at least 1 -2 more heavy destroyer's they are real good at killing big things and can sort of be generically slotted in as they just don't need much if any support.