Southern National had two versions of this livery. the 'Dorset' version shown in the photo, and the 'Somerset' version which the EFE model is based on. The latter had two green bands, the former also had a green roof. Either would be suitable for a layout based in Weymouth/Portland as they did occasionally swap territory! EFE have also done a Leyland National bus in the same livery, and a Plaxton Paramount coach, so along with the Olympian you've already purchased there is scope for a nice small fleet in that livery.
So far as route numbers are concerned, 22A is the original Southern National Portland route number, up to the early 1970's. When NBC merged SN with Western National, Portland routes were renumbered to the 42X series, anything from 422 to 428 being common depending on route and destination. When Southern National returned in 1983, Weymouth - Portland became route 1, with variants such as 1A. However 501 is also correct - that was the open top service to Portland Bill - which EFE also model a version of, in a yellow/blue/red livery which is correct for most of the 90's from memory, and would be another good addition!
Finally, Smiths of Portland operated buses from a garage very close to where the old Portland Station was, including some very old Bristol RELL's until the late 90's, bought from Western National and operated in that livery (Blue/Cream/Red stripes) minus the branding for some years - google Smiths of Portland RELL and you should get some decent photos - these would very local to the model. Again, EFE did a model in that livery, with Western National Branding, which would be appropriate.
Hope that helps
David