Im looking at replacing the front door speakers and keeping the original tweeters. I've looked at loads of 6.5" speakers but what mounting depth is recommended. I don't want to buy a set which then gets in the way of the window etc. I have seen some K2 Focal Speakers but they are 73mm will they fit?

70mm is the absolute limit.

Depends on how you fit them. I have 77.3mm deep 165KRX2s mounted on a spacer. No grilles.