J35 (1958) at a minimum for the IO-470 engine. Anything earlier will have come with the E-185 or E-225 motor. There are some planes with retrofit STCs on the earlier airframes, but I don't think you'll find one properly sorted out for 50k.
My best would be the S35 (1964) if I could stretch it on that budget, which gets a ton of functional improvements, including the seats on rails, modern panel, single wing tanks, and IO-520 engine. HIGHLY unlikely on 50k, but I know of two cases where one was obtained for this money and was not a total POS.
the in-betweeners (K35, M35, N35, P35) are all fine machines and more likely to be in the price range quoted than the S. There were a lot of small and continual improvements in these models.
These are all V-tails.
50k is exposing a lot of debonairs lately too. I would personally opt for a B33 (1962) or later for the center-stack radios and single wing tanks.
For anyone shopping, the book "Those incomparable Bonanzas" by Larry Ball is the best $25 spent to identify all of the little differences in the entire line.
$0.02
- Mike