Charly
I am located in Thailand.
Try looking at these websites for available equipment and prices - these are both reputable camerashops in BKK.
http://www.avcamera.com/http://www.fotofile.net/- Some discount may be offered in-store.
(There is also 7% VAT refund at the airport - but not for secondhand!)
You will get a better EX rate for money in downtown BKK, - Definatly better than you will get in the UK! Though you will need some cash for the Taxi from the airport - unless your hotel provides collection. - you can exchange money inside the terminal with no commission - unlike the UK!
http://www.siamexchange.co.th/http://www.superrich1965.com/node/18There are some second hand camera shops around Yaowarat (chinatown) I have heard - but cannot confirm.
If you fly via Dubai, it may be worth looking for the Isux there - though anywhere you decide on will have local warranty - unless you get international.
Incidentally, where are you going I might be able to give you some locations.
Philip