I wouldn't touch a signature- too much to go wrong. You will basically have to change the whole driveline (battery, RDM etc) to have a reliable car. Finding one that has been completely updated may be tough. Go for a higher vin car with a decent amount of miles. 2xxx should be more reliable but YMMV. Either way plan on setting aside at least 5-10k in case something goes wrong.
Main difference is the SIG's have an exclusive silver mirage color (only 100 made). Basically silver wind with a goldish hue. The interior has Henrik's signature on the front glass along with the # out of 100. Also the interior is a special monsoon tri-tone that couples a dark brown with cream vs black with cream on the normal monsoon's.
Post the VIN here and I will try to find out some information on the vehicle. There are other people that can also look up this information as well. Once again I think it would be foolish to go for an early vin car at this point.
You can read up on all the differences between the trim models here:
http://fisker.ignitedusa.com/en-us/karma/brochure/