I would be quite surprised to get low speed buffet before the stick shaker...even above 20 000ft....
Do you actually operate the 737? this is a regular mandatory sim exercise. You may be surprised but Boeing will not be, thats why they make several references to this is their High Altitude Manoeuvring publications and the FCTM.
During high altitude approach to stall recovery (737 Classic for example) there is no specific mention in the FCTM as to what airspeed you should attain before safe recovery to level flight. A clue is under Holding above 25,000 ft with no FMC. It recommends Vref40 plus 100 knots. eg 50,000 kgs would be 131 +100 = 231 knots. If the stall was due to mishandling in severe turbulence and during the subsequent recovery the aircraft is still in severe turbulence, then you should keep descending to reach recommended severe turbulence speed of 0.73 or 280 knots IAS before levelling out. Height loss will be about 3000 feet no turbulence - or 5000 feet to attain severe turbulence penetration speed.