Later models had left hand shift and 5 speeds .
5 speed gearboxes are unnecessary ,the bike has enough torque to pull 4 speeds and top gear is the same ratio anyway [if I recall correctly] .
IMHO right hand gearchange is preferable ,simpler because there are no cross shafts and it's more natural to use a right hand gearchange .
I wouldn't worry about the year of the bike ,just pick a good one in first class condition .
Later Bonnies are cheaper because they are less collectable ,the most expensive and desirable ones are the "67 to "70 models .
Don't discount BSA and Nortons they are excellent bikes and BSAs seem to be a bargain right now .
You will have to do more maintenance on Bonnies than modern bikes , but if you keep up the effort they will be just as reliable .
Blessed are the cracked, for they let in the light. - Spike Milligan
"When you come to a fork in the road ,take it ! When you come to a spoon in the road ,take that also ."