While you are not looking at a large number of grafts. Some aspects do not change for the clinic. Especially if they only perform one patient per day. You have the setup in the morning, medications, etc, and cleaning of the facility post-op. The staff may be of a similar number also if FUE. You may find a clinic that does perform two patients per day more able to fit your criteria.
600 grafts is not a great deal for a hairline. Is this your first procedure or to refine previous work? With a hairline, the first few mm´s are single hair units to ensure a soft look. As a result, 600 grafts are not going to go far. You should also consider your age. As any hair placed is going to be permanent. Any movement of your hairline will result in needing more refinement. Hope your research is going well.