Not sure about the curtains. Target has some curtains you could try, but I'm not sure how cost-effective it would be for the short term.
With regard to paint, you're better off seeing if you can arrange a time with the realtor to meet a few contractors at the house before closing. I'm a construction project manager and I can tell you that with paint, it is very difficult to estimate a project without seeing it AND knowing what paint colors you want used. "A few different shades" could be anything - different colors for the wall vs the ceilings? Are there chair rails, picture rails, baseboards? Is there molding around the windows, or just drywall? Is the existing paint a semi-gloss, or matte? (Makes a difference, because anything semi-gloss or gloss that you want to be satin or matte finish will need primer first.) Assuming the least possible amount of work, I'd say you're in the ballpark of $10K for a house that size, but you could be at $15K if there's a significant amount of detail work. In any case if you know your closing date, get your bids NOW and get the work scheduled. Any contractor who's any good will need as much notice as possible to get the work scheduled, if you have specific dates when it needs to happen.
Best of luck to you in your move!