Welcome another BEC enthusiast
OK - maybe I'm not advanced builder, but... I had to do my homework because my budget is tight.
I think that rotating engine 90degress and sprocket adaptor it's the cheapest option. I'm making my own sprocket adaptor from old bike sprocket and... steel donut finished on lathe. Then 2-part propshaft made to fit - diy or by professionals.
Reverse is made from "sandwich" reverse gear (mine is laser cut) and starter motor form VAG (counterclock rotation).
Depends on engine - you need right diff ratio to get compomise between max speed and acceleration, but as the diffs are interchangeble you can always change it later.
Remember - don't hesitate to ask, we are all here to help one another