Did the seller indicate why it was a rebuild title? Was it wrecked or Flooded? As far as the smoke from under the hood, it's probably just a oil leak dripping on the Exhaust. In other words, nothing too major. The cost to rebuild the engine varies greatly depending on how much of the work you do yourself and weither or not you want thicker Apex seals, port work, balencing etc. They are not hard to work on and are pretty simple engines. There is nothing to be afraid of with them as long as you educate yourself on how they work and keep up with the service religiously. If this is a car that you plan on modifying it can get really expensive really quick. If you are looking to leave it pretty much alone, then they aren't too bad. Most of them have engine problems because of poor upkeep and not being driven properly. If you want a Rotary to live a long life, it needs to be driving hard on a regular basis. RX-7s that are babied and driven to church on Sundays, die young. They do have some known cooling issues, so budget for new hoses, Water Pump, High Performance Radiator and better electric fans. Also, clean out the Oil coolers as well, or upgrade them to better ones. Then get rid of all the emission vacumm line BS and that should eliminate most of their known issues. Figure anywhere from $2,000-$12,000 for a new motor depending on what you want and if you do the removal/install yourself. Once you get one set up right, and keep up with it, they usually stay right. If the car seems to runs well and doesn't have any issues, then $6,000 is probably pretty reasonable. One last thing, have a compression test done before you buy it. You may have to have this done by a dealer, as a traditional compression testers will not work on a rotary. If the Compression test is good, GAME ON! Enjoy.