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My advice is to look at the whole range of lenses of a particular mount. Find a mount that has good options for the focal lengths you'll eventually want. This way you'll hopefully only need to buy/fuck with one adapter which makes life simpler.
I also shoot with the FD 50mm 1.8 (pic related) namely because I already had a few FD lenses prior to buying my A7. The FD range has lots of gems and the price has stayed relatively reasonable while other lens mounts seem to be appreciating in price now that mirrorless and adapting lenses has become popular.
My buddy has a 55mm (I think) Minolta mount that also takes beautiful photos.