Graham, I have one - Mirex TS for M645 to A mount, which then needs an A mount to (F)E adapter, such as the Sony LA-EA3 I use for Leitax'd older lenses. At the time of buying most such TS adapters were from memory not specific to A mount, but m42, only from memory here, but I had faith Sony will keep going. I found it a little fussy on the a900 but the a7r is superior in many ways that will help: more res, EVF, histo, review, etc. The first universal camera..
I have a number of the M645 MF lenses some of which are very decent. I have not tried any on the a7r (my travel cam!), figured I would sell them and the adapter to an A900/a99 user or see what Sony does for that mount. Many lens aficionados really rate the 120/4 as one that stays with native 35mm format lenses, but they all have a fine draw and tones. The 80/2.8 is another fine one.