Hi- I'm an school expedition leader, having lead several expeditions for World Challenge competitors.
First of all- don't spend your money on the most expensive Gucci kit. Expeditions tend to trash gear rather quickly, and top of the range lightweight camping gear/clothing isn't always best.
Its not what I normally wear, but Craghoppers clothing is great- designed for traveling (hanging loops, quick-drying etc) and is cheap as chips in TK Maxx. The Craghoppers stuff I brought several expeditions is still going strong, whereas some more expensive kit has fallen apart. I've just got back from a four week expedition, and spent 99% of the expedition in a pair of rip-off Crocs that cost me less than a fiver from Shoe Zone!
Personally- I went off self-inflating mats years ago- I've since used a Thermarest Ridgerest. Its a lot bulkier, but straps on to the outside of my pack and is totally bombproof- unlike inflating mattresses, and is cheaper and very light too.
As for the sleeping bag- if you've got the pennies and can keep it dry and clean go for a down bag, but again- you would probably be better off with something cheaper and more robust- ie: a synthetic.
One other piece of kit which I've cherished, but lost on this expedition which I'm completely gutted about is my Tilley Hat. Has been a faithful companion on several trips, and is guaranteed against lost and they will even replace it for free if it ever wears out. I'm gonna have to buy another. Not to be underestimated how important a good piece of headwear is if you're heading somewhere with strong UV/sun.