Leon, for most shooting, pre-set the shutter speed dial & just worry about aperture. You may have to play with it a little bit in low light shooting, but if you do this before you begin shooting, you'll have a goodidea of where you need to be. But if you're hooked on AE & 40 mm focal length, then go for the R3A.
I have seen the Rollei 35 Rf used with a year still left on the warranty for $450 USD & you may be able to get it cheaper. Combined with the 40 Nokton, this is still cheaper than the deal at Robert White. But I must add that the Rollei Sonnar is a superb lens & with today's fast films, it can handle many low light situations.
The 40 Sonnar is a legendary lens, designed by Carl Zeiss & built by Rollei in Germany under license from Carl Zeiss. The barrel is built by Cosina on contract from Rollei, but all assembly of lens elements in the barrel & quality control is done at the Rollei factory. It also uses the renowned Rollei HFT coating, which is the same as the famous Zeiss T* coating. The Cosina barrel is a brass barrel & is from the second generation of Cosina RF lenses, which are a step up from first generation build quality, which is to say excellent. The major upgrade here from a Cosina lens is the coating - absolutely no flare with this lens - & high standard of quality control that Zeiss demands of all its licensees. Of course there is also the superb Sonnar design from Zeiss.