Not sure if you have painted anything yet, but you can also use the death compant squads as assault marines, just with more details. Persoanlly, I don't bother with death company as I don't think they are very good right now, but I have a ton of them as assault marines.
As for where to go with your stuff, adding as little as possible, I would make a list that looks something like:
Stormraven carrying death company/assault marines or assault termies and a furioso dread
Land raider with whatever squad is not in the stormraven
then, if you want more shooting: go get some razorbacks with 5 assault marines in them and a pred or 2
if more assault: add some JP assault marines and sanguinor
No matter how you look at it though, your initial purchase has you all over the map...