Sorry, I forgot your other questions...
Generally, an ETB Amazon Basin will have a pretty mellow temperment. Most GTP's will be mellow, save the Biak locale. Aru, Sorong and Jayapura are probably the most well noted in the way of natural mellowness. Northern ETBs tend to be more high-strung and need a lot more work to tame down, but it's not a HUGE deal. Amazon tree boas a very rare to find that can be tamed down good to the point that I would trust it.
ETB basins "can" get up to 8 feet, but I'd look more at 6 feet. I've never seen an 8ft. one...but I've heard about it. GTPs generally stay between 4 and 5 feet, but may get 6.
ETBs overall are more sensitive to care requirements, so if you're looking for hardy....you may not want a GTP either. lol. They're both very fragile snakes that NEED daily misting and temperature monitoring.
Regurg is really only a problem with Emeralds, which is one reason they may be harder to keep. You have to be very careful not to overfeed. It is better to under feed than over feed. Remember, 3 meals max between defecations.
Price is relative. As long as the dealer/breeder is reputable and the snake is CB or CBB, you should be fine, but I'd rather go with CBB. NO WC EVER!!! These are very prone to dying. Frantz and Signal are my favourite people to deal with, but there are many others. Just avoide Spruce Nubble Farm...they have a good history at ripping people off.
These are awesome snakes to keep, and very rewarding. Hope you can get one soon! They are easy once you do your homework. 