Re: arabian style mehndi.....
well since it's arabian themed then that's probably the best idea. are ok with sittin on the floor as well? if so then that might save on hiring a seating arrangement.
i guess you will need large rugs, a lot of cushions in different shapes, dark fabrics for the decor, some arab style props such as shisha's, lanterns or anything else that your friends and family might have that they are willing to lend, some potted palms, some low tables (especially around where your sitting)...
have you looked at how much it would cost just to have the main bit set up? you could do that and then get your family/friends to decorate the rest of the hall a bit. it would be nice to have the fabric draped from above to give a tent like effect but it might be kind of hard to do yourselves.