£2,500/two countries- Vietnam + Borneo or Guilin?

Would a jungle trek in Borneo be a good idea? It's not like I'm a total newbie when it comes to the outdoors (I go hiking in the more mountainous bits of the UK every couple of months), but trekking in hot/humid conditions is the opposite of what I've done previously. Do you think I'd come across any issues (e.g. finding people to go on the tour with)? I've got Guilin as my backup (and it would cost about the same in terms of flights), but I've had my heart set on the jungle for a long time.

I want to avoid package tourists and 'rahs' (gap yah) as far as possible. Any advice on this?

My budget is in the region of £2,500 (including flights, which are just over £1,000 in total). I don't drink or smoke at all and I'm a dab hand at haggling too.

I have some more questions though:

-Firstly, how long could I stay out there with a little over £1,000 for daily spending? From what I've read around the forum, it'll last me around a month, but I haven't done any proper calculations so the costs could be far higher (I haven't accounted for vaccinations, tour guides in Borneo and all the other 'little' costs that soon add up for example, and who knows- flight prices could go through the roof, and the GBP could crash).

-Will taking everything in a 30L bag be stupid? I go hiking quite a lot so I have plenty of compact + lightweight things (towels, rainclothes, silk liner, clothes). My clothes are all intended for hiking and are lightweight + quick drying as a result, but I don't have any experience backpacking so I really wouldn't know how much space I'd need. The only problem I can think of besides size is the fact that I'd have to leave my Swiss army knife at home.

-Crocodiles in Borneo. They have really cute fat arms, but are bloody terrifying otherwise. My parents would be less than pleased to have a phone call from the consulate about how their daughter got eaten on holiday. Am I being ridiculous?

-Are there any customs/manners that aren't mentioned by the tourist guides? I don't know if it's the Brit in me coming out, but I'm terribly afraid of offending someone accidentally.

-Will a basic knowledge of numbers, greetings and things like thank you/please/a little etc. be enough? (I'd bring language cards too).

-Will the nights be chilly?

Thanks!

EDIT: I forgot to mention- how does everyone deal with money? Credit cards + a fanny pack type thing for cash?