Depends on your citizen number bud, but assuming you're in the first 200,000 then i.e. citizen 1 - 200,000 then you can get into the multiplayer.
So grab the hangar module, now called "Arena Commander" and you get:
Hangar with your ship(s)
Single player free flight mode
Single player v NPC Scythes
Multiplayer V other players (if Citizen number <200,000)
It's pretty rough and ready alpha just now, but it does look fantastic. The arena itself is fairly small and if I'm honest I've enjoyed free flight the most, just to hear/see the ship and eject to see what it's like to float around.
Enormous download though, at least 12Gb.
Worth it? probably if you fancy checking up on progress yourself but if not hop on some youtube vids instead.
On first day of release of Squadron 42 (single player module leading to Public universe) you'll have exactly the same as other players, at the end of SQ42 you enter the PU (mulitplayer universe) where everyone has the ships they pledged for. However although they may have the ships you can get them in-game with in-game money at any point and from day 1 you have the same content open to you.
Edit: No DLC (as far as I've seen), real cash for in-game money is capped per account per month. Not sure how they'll handle it but microtransactions will be used to fund the PU and future development no doubt. I can't see it giving you an edge if done right.