What type of army where you thinking of building close combat, shooting or psyker? Is there a HQ you wanted to build your army around? Is your army for fun or do you want play in tournaments? Since you stated you plan on running a Death Company stay away from the jump packs they are over priced for this unit, I normally run my DC with ten marines, 8 with chain swords & bolt pistol and with 2 power fist & bolt guns (since they have the relentless run they can rapid fire and still assault, a pistol is a waste since you don't get the +1 when using it with a PF), the power fist is vehicle and walker units that get in the DC way since they have to follow the rage rule. In low point game use a rhino so you more control where they end up. Higher point games add a chaplain for re-roll hits and wounds on a charge and put them in a stormraven. I would buy one Death company box just for the all the bits you get with it after that just use basic space marine, I bought a tactical squad along with my first death company box build ten DC marines and 5 Assault marines out of it.
"So fear not and be proud, for we are the Sons of Sanguinius, the protectors of Mankind. Aye, we are indeed the Angles of Death."
- Commander Dante