Is there a target application - wildlife, distant portraits, futzing around?
Speaking from experience, the Takumar 300/4 is decently sharp wide open and improves greatly upon stopping down. CA is plentiful. It is large and heavy and minimum focus distance is something like 17 ft.
Focus action is very pleasant.
I agree with @pepperberry_farm that the SMC A 400/5.6 is a lens to strongly consider. CA is an issue but improves considerably stopped down just a little. Sharpness wide open is just OK but again, improves noticeably even half a stop down. Wonderfully long, well-damped focus throw for precise control, MFD is less than 10 ft, nice color and contrast. Probably my favorite lens ever.
Given how low-priced the various xx-300mm consumer zooms have become, it is tough to recommend a legacy 300mm lens based solely on price.
Tak 300/4
A 400/5.6