When I'm building a list I like to think about what I can handle as well. At 500 points there's only so much you can bring with SM but there's a lot other armies can bring; you could be facing more than double your number of models more often than you'll be facing something that needs a lascannon shot from 48" away. Chances are you'll likely be playing on a 4x4' area instead of 6x4' so mobility and range won't be huge issues to overcome.
Really you just need some flamers to help against large squads, missile launchers are nice because they can do good damage to a variety of targets. Your Captain is likely going to be a combat character since he doesn't buff squads in a support role in any other way really, I'd suggest looking at some wargear other than a bolter for him since he won't be able to charge after firing it (nor will his squad thanks to the rapid fire weapon rules).
Do you have any experience with magnets, or can you easily swap the Dreadnought arms? It's been a long time since I've bought a kit and despite owning four of them I can't seem to recall how they assemble.