if one can find, get the whole assembly including P valve, vaccum valves, the ratio of oil flow is different for rear disc brake and rear drum brake so use correct jets - once again getting the complete assembly makes things easlier than trail and error.
about handbrake get the cable with proper ends, people can remove drum brake end and clamp disc end , wire is normally slightly shorter for disc brake.
about hubs , rotors, get them cleaned , use new brake pads all four wheels helps getting stable braking.
Most mechanics in Pk come up with their theory, dont go along with till yr satisfied their idea matches with yours to avoid trouble later.
dual brake boosters also help
last but not least dont buy dusty, rusted stuff. in Pk everything gets dirty very fast , some parts are not to be left open in dust but sellers dont care.