A general rule of thumb -- well-maintained 6.3s tend to run $15,000 and up. Below that, you are going to pay many times the purchase price over, in deferred maintenance costs, to make the car roadworthy.
There is nothing better than a well-sorted 6.3 that is operating according to factory specifications. Unfortunately, 80%+ of them, AREN'T. So owners never experience the true joy of a 6.3.
There is nothing worse, or more painful, than a cheap 6.3 (under $15K). I don't care how good of a deal it seems, or how many "individual" parts have been replaced. If the car hasn't been attended to by a competent mechanic (and most MB dealerships ARE NOT competent when it comes to a 6.3 -- few have mechanics on board experienced at working on vintage cars), it is not worth buying.
Also, if the car has resided anywhere in the world outside of the West Coast of the USA (Washington, Oregon, Idaho, California, Nevada), the US Southwest states, or the US South states, or the Australian desert, there is unquestionably rust somewhere on the car. This is a statement made with complete confidence. If the car was ever driven in the winter, and the area where the car resided at any point in its life used salt on the roads (most of the US and Europe), there is rust. I don't care if the car was only driven on salty roads for one winter. It has rust.
Download the sheet at my website (URL:
http://homepage.mac.com/gerryvz/checklist.pdf ) to do a comprehensive check on the car's condition.
Owning a 6.3 is no easy thing. It requires commitment, patience, and most importantly a large wallet. You will get thrills beyond your wildest dreams from a well-fettled 6.3. But it is always best to spend the money up front to purchase a car that has benefitted from someone else's outlay of cash, time and labor.
Trying to revive a moderate running car, not to mention a basket case, is an exercise in frustration and most people end up selling it after thousands of dollars of outlay, to little benefit. Most people don't want to spend the $30-80,000 required to truly restore a 6.3 basket case. It happens all the time -- people get "6.3" on the brain and buy a $6,000-10,000 car, and after a year or two realize what they have on their hands. A money pit with little light at the end of tunnel.
Hope this all helps. Happy Thanksgiving.
Cheers,
Gerry