$500 is a decent price for a parts jeep. I buy those all the time. In fact, my wife is getting a little pissed at the jeeps that are littering our woods.
If you ever want to register the darn thing, you'll have to simply be patient and wade through the red tape.
What I have done in the past is, just tear the friggin VIN tag right off and toss it. Then totally rebuild the whole tub, and tell your MV dept in your state that it was a restoration. They'll grumble, and probably ask to inspect the vehicle, but in the end they will issue you a new VIN.
Now bear in mind, this is ONLY if the tub is worth saving.
Also, many states allow you to register your Jeep as a farm vehicle, and it is a flat fee. In NH here, you can do this but you are only allowed to travel 12 miles from home. If your state has somethign similar, you may want to look into this.