Id take the clothes im standing in and have my gear sent to new-Zealand.Ive done 3 long trips in that area,first time took way to much,second time onwards got it about right.Take next to nowt and buy as you go along.
Only time youre likely to need a jumper is for the air-con on the coaches.
Somethink along the lines for my last trip would be
1) 1 pants 1 swimming cossie.1 easy wash top 1 trousers (zip offs) 1 socks (optional)1 lightweight warm layer.1 umbrella.Snorkelling gear.
Only times I was cold were in the mtns of Flores (Buy locally) on the highest mtn in Thailand (cant remember the name offhand) and on the night time air con coaches.
Dont be like 99% of backpackers travelling with half a tonne of stuff they,ll never need.Go light.
Excellent destinations on the way down are the Perenthian islands,tioman Island Koi samui ( 20 years ago) and before the tsunami phi phi,Krabi area and phan nga bay.Havn,t been since the tsunami.
I spent 7 days trekking in N Thailand booked locally at a guest house,cheap as chips but on some of these treks leeches were a pain,make sure you dont go to near the front of youre walking party.We always seemed to end up staying in the local opium dens which are preatty colourful places.
Second trip I done was in the western penns rainy season,if youre thinking goretex forget it just enjoy the slight cooling effect of the downpours on youre skin.
While on the subject of coaches in Thailand dont book the crappy backpacker ones that come out of the ko san road,not a thai in sight,full of backpackers with shit food and cramped as hell but waddle down to the central thai state bus station to go on there luxourious coaches with loads of room,fantastic food and you will probably be the only non- thai on the coach.Great fun.
Have a great time,its a magical place to travel and very easy even for a beginner.