E90 320i Tensioner and pulleys

hi all,

just want some advice please.

my car has the N46T engine with 160 000km
i want to replace the belt pulleys and belt as preventive maintenance.
I want to know if i can just replace the wheel/pulley on the Tensioner and not the whole tensioner?
BMW sells the complete unit, ie tensioner with pulley, which is expensive and in my opinion not necessary as the tension seems fine.

Should I just replace the whole unit or not?
If not, where can i buy the tensioner pulley in JHB?

Thanks