Better legacy/vintage manual focus lens system to invest in for NEX

I would challenge the assertion that RF lenses are "much" better than SLR ones. Admittedly, the Contax G 35/45/90 are perhaps the best as a group of any 3 such moderate focal lengths for 35mm. However, the superiority lies not so much in design as in the superb specs and quality control exercised by Zeiss. Above 35mm, there is little difference in optical design between RF and SLR lenses. The 45mm G is a double Gauss like all normal 50ish-mm SLR lenses.

When you get down to individual lenses, the 35mm G is about the same quality as the best SLR lenses, and the sharpest 90mm ever made, according to Photodo, is the Tokina 90mm F2.5, an SLR lens (and an optical clone of the Vivitar Series 1 90/2.5, probably the sharpest lens I ever used).

Where RFs had an advantage in the film era was in wide angles, because they could use symmetrical designs rather than retrofocus ones. However, symmetricals do not perform well on digital sensors.