Here's my thought. I was once seriously looking into going on a trip throughout SE Asia. I researched Everything and had everything planned except for my money. Things came up so I ended up not going, but if I have the opportunity again I definitely will, its an amazing place. But if your parents are the ones who are paying, I would take them up on Europe, simply because it is Much more expensive and it'd be better to see it, if you didn't have to pay. That way, later on, you can pay your own way to SE Asia if you want (super super cheap, you can live in Thailand for a month on $2000 - less even) and you won't break your wallet.....just a thought....
oh and i'm not sure about your other questions, as I have never backpacked before, but I think maleficent has the general idea. I've always been told by friends who have backpacked, that you should lay everything you intend to take on your bed and then cut it in half. You don't need everything you think you do
I would do some searches on sites with info for first time backpackers. I know there's lots out there as I came across them while doing research for SE Asia....
Let us know what you decide