I am not sure what the seller means by air-bag issues. Did they deploy at some time for no reason, where they part of a recall but never serviced?

I know you are considering an 85' 500 SEL like mine but can tell you that the day mine was sold off the showroom floor, it came with a recommendation to check and service the air-bag at 10 years. I doubt that ever happened. Even though seatbelt tensioners and air-bags work to the same end, your safety, they work independently. My 20 year old tensioners will still lock up if I deploy them too quickly but I would not bet money that my "depowered" air bag(pre-1997) will do the same even if my SRS check-light says it will. Does any manufacturer recommend a longer interval? This is a pretty sophisticated function to ask for it to be reliable much longer than that without inspection.

Compare them both, 85' 500 and 90' 300 very critically before you decide to walk away from either one. As far as airbags go, they may both have the same issues. OD miles and documented maintainence, including air-bags, are much better barometers of worthiness.

Once again, good luck