Depends on the size of the engine, bigger engine = more offset.
1.5" is fine for most engines commonly fitted. 75-90HP range,
smaller engines theoretically need less offset but its not critical, if the transom is already drilled i would use the existing holes.
What size is the engine? not the 35hp you have on your details i hope? that wont be very exciting
you will need a fine pitch prop on that to get anywhere at anything other than displacement speeds with any weight on board, it may take a while to get out of the hole if you have the flooding hull