I'm now playing Skyrim for the first time, i'm level 30 atm.
And was wondering if i buy Dawnguard now, am i gonna miss some content is skyrim.
Is the Skyrim main quest still viable?
What you think should i buy it now or wil i have more fun to first play more content of skyrim and buy and install dawnguard later?

you could go with it when ever ya like. only came across two quest that share there doing with the main quest line. but the doing of it would only have you keeping said item gained in your inventory longer.

the dawnguard quests line wont start till you make the effort to begin it save after lvl 10 random attacks by vamps

so they run prety independent of each other.. i would grab it before you start another toon it will be less likely to mess with your save file having it there from the start of things then adding it too your present lvl 30 toons save

you can finish both straight hough either or mix things up do both together. only difference is stated above. untill you chose too begin things and maybe random npc talking of the vamp/dawnguard

I'm jumpin between Skyrim and Dawnguard quests, have finish the "Awakin quest" and should soon start on the "New order" quest.

NOW!

I say get it now. Why not enjoy all the content skyrim has 2 offer all at once?

I bought it a few minutes after it was released ( for the PC of course )

do you have money if so you have two choices 1 buy dawnguard 2 give the money to me so i can slap you with it for not buying dawnguard

Start it whenever you like, it doesn't interfere with anything.