Ok, I am bored...so, I will do what I can to help.
It's a very cute outfit, one that friend was going to do last year, but never got around to. But...
re. pattern: search simplicity.com or mccall.com (has vogue, butterick, mccalls ) or kwiksew.com. Or hit your local fabric store and look thru the many fabric books they will have. Look for the chinese style tops. They're not hard to put together, and a mandarin collar is easier than you think. Modifying an existing garment woud probably also work. Whatever you choose (pattern or existing) will require modification, so you can't get around that.
However, I still wouldn't know what to do about the intricate gold, purple, and blue designs on her top and pants. Fabric paint tends to chip off after a wash or two.
I don't know what brand of fabric paint you are using, but I have never had trouble with mine. It's... um...shoot, I forgot the name, and it is back home in my craft stuff. There are plenty of different kinds, make sure you ask someone at the store who has fabric paint experience. And personally, I think paint is the best idea for the designs, because it can go on the fabric well and look smooth, and comes in lots of colors. Usually if you iron it after, the color will become permanent. And as long as you're not planning on wearing this garment every day and running around and sweating in it, then I don't think you have to worry about the paint chipping off or something. The other option might be iron on transfers, but IMHO, they don't look as good because the color isn't as bright. The gold trim I would use gold bias tape, handsewn on. Takes time, but looks way better.
I also need suggestions for how to fix the sashes at her side and what fabric I could use.
It looks light and flowy, but not shiny or see-thru, so I would go with either a light cotton or silk. Silk would probably 'flow' better, though. As for attaching them, it almost looks (from the pictures) that you could wrap it around your waist, and tie the sash at the side, where the little slit in her top is. That, or sew it directly to the pants. Or use velcro.
What kind of material could I use to make her tiara/headpiece?
Hmmm.... crafters choice interfacing on the same blue fabric used for the shirt with the designs painted (or handsewn) on and attached to a headband. Or really thin cardboard, like from cereal boxes. Or paper mache over a wire frame... experiment until you find one you like.
Hope this helped somewhat. I am too sleepy to tell.