and also some sort of logging to see time stamps and cards coming and going.
That sounds like a very good plan.
SD would be good or maybe an ethernet shield, external logging and access. Maybe some kind of remote locking/unlocking.
Someone rings you up cos they don't have a card, you then open the door remotely. Maybe have a network webcam set up on the control page too.
The next things I would like to add are both lock out times E.G. - card1 only opens the door from 7am-11pm. So card1 could never open the door at 2am.
RTC, bit of code. No worries. 
Mowcius