The basic difference between the R2A and the R3A is viewfinder magnification and the framelines shown. The R3A shows framelines for 40mm, 50mm, 75mm, and 90mm lenses at the same magnification that you see without optics. So if you leave both eyes open, the image through the viewfinder and the other eye match exactly and you can get the effect of the framelines floating in space. I love this feature, and it's the main reason I bought the R3A. With glasses, the 40mm frame may be on the edge of visibility in the finder, or perhaps just outside it. With contacts, you should be able to see the whole 40mm frameline. I can just make out the 40mm framelines with glasses that fit close to my face, but most people can't see the 40mm framelines in the R3A while wearing glasses.
The R2A viewfinder has framelines for 35mm, 50mm, 75mm, and 90mm, and magnification is 0.7 lifesize, so you get different sized views through the viewfinder and your other eye, making it hard to frame with both eyes open, and a feeling that's more constricted (like an SLR) than with both eyes open.
One other thing that will matter in making this choice is which eye is dominant. If you're left-eyed, the 1:1 finder on the R3A is no great advantage, as your right eye will be blocked by the camera body.
Neither camera has finder frames for your widest 28mm lens choice. You'll need to use an auxiliary finder for wider than 35mm. The Cosina Voigtlander wide angles shorter than 28mm are supplied with auxiliary finders. 28mm and 35mm finders, and a mini 28/35 combo finder are available separately.
The R3A and R2A are on the same chassis, with the only difference in the viewfinder magnifications and framelines. Build quality is the best in the Bessa line, perhaps a bit better than the R2, and definitely more rugged than the R and L.
Nearly all Leica mount lenses should work on the R2A and R3A. Exceptions might be some of the extremely deeply recessed wide angles, and perhaps some of the lenses with auxiliary finders (sometimes called "frog-eyes" in English) that modifiy the field of view through the Leica cameras. If the "frog eyes" are removable, such as on the dual-range Summicron, you can still use the lens without the auxiliary finder. You can mount bayonet M mount lenses directly, and Leica screw mount lenses will fit with an adapter from Leica or Cosina Voigtlander. I think Cameraquest has a list of Leica lenses that might be too close to the shutter curtain or obscure the metering cell to be used on the two cameras.
Hope this helps.