Is it worth it to book before I go?

Hi all,

I will be in Thailand in November 2009. I fly into Bangkok on 4th and leave for Phuket on 12th. I would like to do a couple of tours in Bangkok and then check out Ayuthaya and then spend some time in Kanchanaburi but dont really want to keep travelling back to Bangkok to start the next tour.

What I would like to know is whether I should book all my tours from here or wait till i get there? How easy is it to find tours in Bangkok, Ayutthaya and Kanch? The tours I have looked at here are expensive and dont include everything I want to do but if I dont book, will I miss out on doing the things I want to do, that is included in these tours?

Would it be better to hire a tour guide there to just do the things that I want to do? Is it easy to find one?

Please help!

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