Cannot speak from persnal experience however I have researched these as a couple projects were available locally some months back -
1) Expect Model A like performance (I have had a lot of American prewar cars - essentially the 170 is a flat four simillar in design and HP as the 40 HP Model A engine.) So not a highway car and in some settings, could struggle in traffic. These were designed for European back roads not the Autobahn. 40 - 45 mph on top end.
2) Postwar examples of the 170 are very simillar to the prewar car but little or no structural wood in the body which looked simillar but was in fact different after production resumed postwar. A plus as I do not believe wood kits are available for these. Go with the postwar car if you get serious. Here is a website devoted to these: http://www.170220.org/
3) Here is the big one and this came from a buddy who owns a bigger chassis prewar MB; many of the suspension parts and such are shared with pricier MB models like his, and therefore are very expensive. Not much is available for these from parts vendors.
4) Harry Magno I believe, out f MA knows these cars and restores many. I would get contact info for him from my buddy via PM if you get serious, if you cannot find via google.
5) For postwar, if you can swing/find the 220 it is a much better car in terms or roadability.
At the end of the day these seem comparable in price for a real nice car as a Bentley R Type but not as powerful and a more limited market. They do carry a lot of style queues simillar to the larger cars though - great for the fan of larger early MB but not able to spend the cost of a nice house on a hobby car. That said, an open 170 is worth a not so small fortune these days, I think the sedans are likely where they will be for a long time though.
Gullwing in NY & Bev hills Car club usually have one or two hanging around in inventory.