There is a pulse module in the starting circuit it could be faulty, a corrosive connection near the starter, there is a ground under the car on the drivers side that connects the frame and engine; a multitude of possibilities. It could be a bad crank position sensor if you did not use a Bosch as a replacement. You can download the service manual from this forum; thank you to onehundred80 Click service manual
$250 for a scan is too much. You can purchase an icarsoft MB II or a Foxwell nt530 with Mercedes software for under $200. Both are just for Mercedes and work well on the Crossfire; Just manually enter the car as a SLK 320 or for a little bit more you could purchase a Launchtech scanner that will work on most cars domestic and European.
Water in the lower trunk area is a sure sign of SKREEM problems. Use the search box in the upper right corner of the forum and search water in trunk. For about $50 you can get a fuel pressure gauge and the fittings from Needswings ( that is what ala-xfire was exploring)