You could use for instance the MOPS- buffer (also a Good Buffer), as it has a pK of ~7.2. But keep in mind that carbodiimide coupling needs an activation of the carbodiimide by a proton. If you work under alkaline conditions, the reactivity of the EDC decreases, the NHS becomes more and more deprotonated and (important) the formed NHS ester is hydrolized much faster by water. I think people preferred pH values of 6-7.

Hepes could also work. We work with NHS-esters and had never problems with Hepes (although is has a free hydroxyl group, that could potentially react, this was never observed)

Thank you