Hi there,

I donīt know much about Belgian bikes. Crankset, brakes, seat, front hub and Shimano parts do not look period correct to me.
As you say, the frame material is probably Reynolds 531. Are the threads French, Italian or British?

I wouldnīt sandblast it. This might remove too much of the steel and round the edges of the lugs etc. Better remove the old paint with chemicals and check just how bad the rust is.

Bonne chance!
NR