Very nice collection, John. The early Airsporters are now very good value for money so it is easy to be tempted by them.
I think your collection could do with an Airsporter 'S' or Mercury 'S' (a Mercury Challenger is just too rare a beast nowadays), and really an RB2 would be good in there as well.
However, if you are wanting to go the budget route, there are many Marks of BSA Meteor to consider, all of them great VFM as well.
The two BSA junior guns that might be interesting to add, but tend to be a bit overpriced, are the radical but splendid BSA Buccaneer and the toy-like & cute BSA Merlin.
However, perhaps you have enough BSAs already! That SuperStar is a beautiful rifle, but they did make it a bit deep in the fore-end to be truly graceful, a bit of a porpoise-shape.
I think you ought to look out for a nice Mk 1 Meteor, with the metal end-cap and the grooves in the fore-end. Very elegant!