As with any old cars, unless someone has seen it (and can accurately assess the amount of work) no-one could tell you an exact price.

Its a pretty rare beast if it is a Sandman and depending on exact condition the price could be anywhere. Obviously you want the most you can get, in my opinion the best way to find the market price is to list say on ebay (WITHOUT RESERVE) starting at the lowest price you'll take. Only accept bids from people who have seen it or at least contacted you to discuss. Worst case is doesn't sell, or sells for that price. Upside you get a bidding war from genuine buyers.

The point I make about reserves is because I know sellers who say put the reserve super high to 'test the market' but what they get (IMO) is many tossers willing to make a bids under reserve obviously with no commitment to buy (then if they hit reserve by mistake you see bid retraction). Doesn't give a realistic price for anyone.