UNLESS you are only going to spend your time in capital ciries, a small 100-150cc is a BIG mistake. The ideal bike size is a 250 -400cc, here is hwy...
a) You can double-up (and even if ur travelling alone you're going to want to at times... especially if ur a guy
) and allow for luggage and still hit good touring speeds.
b) If you get a DRZ 400, KLX or XR you have a bike light enough to tackle hill climbs, river crossing, jungle trails etc... tha a big BMW tourer or a 650 can't handle.
c) MOST parts are interchangeable and accessible
d) It's still a big enough bike to attract alot of interest and cultural interaction when you get into the countryside, take a couple of rice farmers or kids for a spin and see if you will be allowed to leave before you've attended half a dozens weddings, birthday paries etc...
e) Not ideal but still a small enough bike to commute the big cities on.
I prefer the 400cc range (XR, DRZ as the ideal size).
ALSO, as a jump off point Cambodia is the ideal. because:
1) Business Visa on entry (just write businessman and tick the Business Visa Box when you arrive) means registering a bike legally in your name with photo ID costs you $26, and you just use a guest house receptionist to do the footwork for you... NO HASSLES whatsoever. You can then enter Thailand, Laos and Vietnam with a legal bike with photo ID.
2) Bikes are about 30% cheaper than Thailand, Thailand has 100% tax, Cams has standard $500 for over 250 cc, $250 for 250cc so they imported more cheaply), Vietnam simply has hardly any bikes available so your up for $2500 for a 20 year old Baja or DR 250.
3) There are a plethora of DRZ, XR 400-650s, KLX XR DR 250s, WR/CRF450, TransAlp400s for sale because its almost all an expat market...
Most bikes are sold via notices but you can try Jobs and Classified Ads in Cambodia - Bong Thom Dot Com
for bikes or Khmer440
forum to confirm how bloody easy and cheap it is to get legal!!!