On my next visa run to Laos I want to try taking Nok Air's DMK to Udon flight which arrives at 7am if not late. Normally I come mid day and take the "limo" service (a minivan) to the border. Will that be an option at 7 to 7:30am or do I need a taxi?

Maybe not much help but anyway..

If direct does not pan out. Grab taxi to central plaza. ,200baht.

Minibus (easy trip) to nong khai. 50baht. 

Or yes personal taxi from udon airport to nk. I haven't done that. 

Udon Thanni ? , the mini vans start at about 6 AM , although the still half a sleep driver stops off for breakfast and a coffee along the way