I am sorry for this question was probably asked a million times but...

I am thinking about getting dawnguard but I haven't started the main quest yet as I like to leave it right to the last thing I do

Will I be at a disadvantage? Should I finish the main quest before I start Dawnguard? Or will it not matter if I play it through before the main quest?

Thank you for your help

You can start Dawnguard at any point in the game, the only thing is you will get something you need for the main quest early, but most of us had that long before we needed it even before Dawnguard. Dawnguard would have an extra surprise if you have at least killed the Whiterun dragon, so you can fight dragons.

If you have not discovered (from the game) who you have to kill to resolve the Main Quest, and the next step then you do not have what you need for Dawnguard.