buy in Vietnam
If you wish to travel all of SE Asia on the same bike you will probably have to buy it in Vietnam. As I understand it you cannot enter Vietnam with a foreign registered bike, even one from another SE Asian country, but on a recent cycling trip to these places I met a couple of people who had bought there bikes in Vietnam and had no problem entering Cambodia or Laos with them.
The most popular bikes in SE Asia are Honda wave 125's, tough, reliable and you can get them fixed anywhere. I did hire one on a couple of occasions and found them fine but the seat got a bit hard after 100 km. The two bike travellers I met had both bought Minsk 125's which have great "character" but use more petrol and are difficult to get parts for outside Vietnam, there were several for sale in Hanoi backpackers for about $250 each with pannier frames.
If gaffer tape doesn't fix it then you haven't used enough tape