No need whatsoever to book a bus, just head over to the Southern Bus Terminal. Buses depart every 30 minutes these days. Minivans also depart from the same terminal, there are also frequent minivans from Mo Chit.
Check with your hotel. Typically a transfer via car is about 2,000 to 2,200 baht.
I should have added the trains which depart from Thonburi at 07.50 and 13.50.
We decided to book a guide that will take us to Kanchanaburi and visits some sights along the way as well as take us on tours for the few days we are there.
Thanks again everyone for your advice, greatly appreciated
Make sure the guide takes you to places that YOU want to visit. Be wary of restaurants he/she suggests to you, and if you don't want to do any shopping then ditto.....Guide or not, YOU still need to do some of your own planning.
Just to confirm Judy's quote, a driver took me yesterday from Kanchanaburi to the BKK airport for 2,200 baht. I didn't try to negotiate because it seemed like a fair price. The journey took about 3 hours. This was Sunday, so not sure if it would have been longer on a weekday