well you can get really nice boats from HCMC to Siam Reap - but they are about $4000 USD for the 7 day cruise
I would suggest you do a tour from HCMC to the Mekong Delta - you can google them there is a nice 2 day one that finishes up in Chau Doc - you then have a couple of choices of boat to PP
I recommend staying at the Victoria Hotel in Chau Doc - and taking their boat -
but other boats are available
From PP there are flights and buses and some boats to Siam Reap - but it will depend on water levels - if near the end of the dry season the boats might not be able to get all the way to the port which is 15 or 20 k south of Siam Reap
Enjoy - it is a wonderful trip and Cambodia is amazing