An 21x12 should do it. Should turn around 62-6300
This is an emhw Ulti? I have one as well. Mine should be <84db in 7m with 3W56 twin and engel 3blade 22x10@7100umin. Motor is soft mounted...
hints:
Make sure the wing cords are tight and attach felt (or something like this) to the wings and the elevator. Another noise source is the upper wing canopy since the steel staves may get play and produce noise.