Gearbox replacement

I have joined from Australia and I’m hoping for a little guidance. I have a BE1 auto. The auto box needs rebuilding or replacement. I’d always rather had a manual and was thinking if going to the trouble of removing the box and rebuilding why not take the opportunity to replace with a manual?
Do I have to source a K10 manual box or will others fit?
Am I better to replace engine and gearbox together? In which case can I source something from a later model and will it fit?
many thanks
happy new year!