Get a +2 mithral breastplate (total armor bonus of +7, and it's light armor since it's mithral), which you can later get enhanced with the Shadow ability and the Silent Moves ability (it'll be a while later before you can afford to pay someone to do that, but it'll help offset the small armor check penalty on Hide checks and Move Silently checks).
With this, you can also afford to get a +1 darkwood shield (soulknifes are proficient in most shields, and darkwood eliminates the check penalty of a heavy wooden shield). This will add another +3 to your AC.
Also, you should be able to afford a +1 Ring of Protection (+1 deflection bonus to AC).
The armor will cost you 8,200 gp. The shield will cost you 1,257 gp. The ring will cost 2,000 gp. Together, this will cost 10,457 gp, leaving you with 1,543 gp remaining.
As for weight, the armor will weigh 15 pounds and the shield will weigh 5 pounds, while the ring will be negligible. The armor's maximum allowed Dex bonus to AC will be +5, and it will incur an armor check penalty of -1, with no speed reduction since it is mithral. The darkwood shield will incur no penalties but for arcane spell failure, which doesn't matter to your soulknife.
With the remaining GP, you might buy a light crossbow. Along with 20 regular bolts, 5 adamantine +1 bolts (magic +1, not just the +1 inherant to adamantine; they don't stack, but the only purpose of these arrows is for overcoming damage reduction), 5 alchemical silver +1 bolts, and 5 cold iron +1 bolts. The adamantine arrows will cost you 100 gold and 1 silver each (500 gp and 5 sp total), the alchemical silver arrows will cost you 48 gold and 1 silver each (240 gp and 5 sp total), and the cold iron bolts will cost you 92 gold and 2 silver each (461 gp total).
Altogether, the regular bolts together with the special bolts will cost you 1,204 gp, and combined with the cost of an ordinary light crossbow, that's 1,239 gp spent on your ranged weaponry. The purpose of the crossbow and special bolts is to overcome Damage Reduction in case you face anything especially tough, like a werewolf or demon or other such beastie with special-material-based Damage Reduction. Your mind blades will already overcome DR X/magic, and you can already throw them a short distance, but they will not ignore the DR of some critters, so you might want to be prepared for that by carrying the crossbow and special ammunition. The regular ammo is just for when ya need to shoot something far away that's trying to snipe or dive-bomb ya.
If you do get the crossbow and ammo I listed, along with the defensive items I listed, then you should have 304 gold pieces remaining to spend. That's what you'll use to get your basic supplies and gear, like a backpack, trail rations, water skins, etc. Also, you should probably buy one or two flasks of acid, and one or two flasks of alchemist's fire, if nobody else in the party has fire and acid attacks readily available. Ya never know when you'll need to finish off a troll, tendriculos, or other nasty regenerator. .....or just set stuff on fire, or melt your way through a lock/obstruction.
