*balloon size, duration, and how many repetitions
*how he'd done at finding issues in the azygous
* is he collaborating with other ccsvi docs or attended any of the conferences or events to learn from each other
*years of experience in IR and what sort of work he's been doing (has he spent much time in the necks?)
* what is the length of his waiting list
* what testing does he do before the procedure
* what anticoagulation
* how long would he take to get back to you or what would be his advice if you experienced restenosis or a return of symptoms or any complications in the months post-procedure (some docs respond much faster than others)
* is there any risk of an IRB shutdown, should you make refundable travel arrangements (the wave of shutdowns may seem like its behind us but once burned, twice shy)
Just some thoughts, also on your end, if you have already decided to go with this doctor then you mostly need to make sure none of his choices will be dangerous for you (I see danger in inadequate anticoagulation, inexperienced docs, the use of stents, and no response if you experience a follow-up complication) then I think it's best to trust your doctor and let him do what he knows how to do.
Congrats, if you are going forward with treatment, wishing you the best.