From my experiemce, the key to winning is to know what your opponent might use and plan accordingly. On maps that are not terrain heavy, melee is probably not a great option, you might find that you don't have enough cover to weave through before getting shot down. I find that flamers have good use as a counter charge device and not particularly great against a shooty team.
I myself play guard, so the unit costs are never an issue for me, but considering you're not going to be able to recruit marines without spending promethium, I would try and squeeze as many power armor bodies out of that starting 1000pts as possible
The Heavy Bolter is a great weapon, but in most cases the amount of fire is not necessary. I would suggest swapping it out for the Autocannon, it's only 1 dice of sustained fire, but it is a more reliable weapon and fires much more powerful rounds at S7 AP-3 and D3 dmg. It is also more affordable, allowing you to further upgrade models.
Melta guns are also similar to flamers in the sense it's not liable to see much use against a shooty opponent, they will be smart to give it a wide berth and focus fire on the gunner before he can get into range. But apart from that, they're going to decimate whatever they hit.
If you plan to use any unreliable weapons like a plasma gun, You definitely want to be giving them either extra ammo or a red-dot (ideally both), You want to make sure the weapon can get it's blows in.
Cultists are very mediocre in a melee fight, unless you're fighting other cripples (Tau, Guard, etc) so giving them any additional melee weapons is a little questionable, usually I would be using them to soak damage away from my marines, so I keep them pretty barebones.
Hope this helps a bit!