Another consideration would be some of the German/Belgium guns from the 20's and 30's. Granted most of the chambers are short but if you are selective there is usually enough steel to allow for enlarging the chambers and forcing cones (short shells are available but you said you have a good supply already). Usually choked very tightly which can also be corrected.
They are generally lightweight. Many have cheekpieces which you may not like but also correctable.
I've got two that I've used for many years after extending the chambers/forcing cones and choking IC/Mod.
My opinion they are well made and of stronger design than a lot of currently available guns.
Certainly within the budget you've mentioned.
