No idea how you managed to get "whisper" reamer as they are on the list of things non-exportable, don't need to know, but hope you do get it in. ANYTHING for 50 cal is NOT cheap anymore due to the hassle created by the US banning the export of EVERYTHING 50 cal. Most of us have resorted to sourcing barrels from Europe now, as we run out of previously imported barrels,since the US put the export ban in place.
There are I believe only 3 Canadian makers for 50 cal actions, PGW , Gaulin in Quebec, and my shop making actions, all of which are well over the $2000.00 mark for a bare action.
The 500 whisper could be built on a CZ550 action as they are long enough to make a single shot from, and are easy to work with. The P17 may have length to work. You won't find any commercial action that will work as a repeater inexpensively, due to the sheer OAL cartridge dimensions