You don't need a new wheel. You need a cog and spacer kit and something to tension the chain (assuming you don't get lucky on the whole magic gear thing - I never have). It sounds like your existing RD is broken/seized and won't tension the chain now. If so, you may be better off getting a simple tensioner. This spacer, cog, and tensioner kit has all the basics for under $40 (and the DMR STS tensioner works very well).

If you can rig up something to tension the chain, then you only need the cog and spacers which will set you back about $15. Your existing RD may work if you can get hit it with some penetrating oil and get it moving properly. If you have any mechanical skill, a tensioner is not hard to fabricate (I've even seen an old jockey wheel bolted to a bent up kitchen spoon). All comes down to what kind of value you put on your time.