i have acute spinal nerve damage and my mattrice is terrible,it hurts my back so bad.
like you i couldnt afford a new mattrece so what i did in the mean time was go to pound stretcher [a cheap shop in the UK] and buy a memory foam mattrece topper-it was only £15,its not perfect but its better than not using it i can tell you that.

as for the smell,it might need airing,could you leave it outside on a day when theres no wet weather? or leave your mattrece exposed and your windows wide open when your not in the bedroom.