If you do everything it needs, a full restoration as your title says it is going to be close to a $200k endeavor in the U.S. But the good news, is a 280SL can hold that value. Concours restored 280SL bring over that, even after selling fees. I know you don't intend to sell, just answering your question and letting you know there is little risk at current market valuations. Also plan for 2 years time if you are working on it almost everyday.
The most expensive categories will be welding sheet metal, body prep, paint, chrome. Engine and transmission are 2 of the easier items to facilitate.