Here are the items that typically require replacement as these cars are approaching 20 years old and historic maintenance may not be known. When searching for parts use 2003 Mercedes SLK320...they aren't expensive and not difficult to replace.
1) RCM will need to be pulled and sent to https://www.facebook.com/groups/202200410510109/user/1232522066/?__cft__%5b0%5d=AZV0GVsBXYtWNsybsRm8ZuDimYeRdWSqp90MKbqheuBMvUokczvTH_HLFze-KwUYyF1LaNfYcM8McNmQC2Dnvpn9k8BY0f2GwBaOvTJhrSt6nyirLtm5-wC9NojpkfbdzGdTCjBeM5pH2Vbn1oqN5Ivb&__tn__=R%5d-R
(tighed1@hotmail.com), the RCM doctor will test all the relays and replace as well as his "horny" fix then buy a new backup RCM
2) Replace the crankshaft position sensor with a Bosch unit only https://www.fcpeuro.com/products/mer...sor-0031532828 . This sensor will go out and the car will stop running...let the car cool down. An option is to pour ambient temperature water on the CPS (The CPS mounts in the tunnel portion of the engine block where it meets the bell housing so you are not pouring water directly on the engine block but pouring the water straight on the CPS) on it to cool it down so that the car will start. Youtube showing the replacement
3) Replace the idler pully, tensioner, and serpentine belt (FCP Euro has a bundle for these https://www.fcpeuro.com/.../mercedes...erpentine-belt
4) Flush/replace all fluids
5) The sticky key repair depending on the date of manufacture of the 2004 to mid-year 2006. I believe you're good if manufactured after August 2006...you can find the fix in from the Files here https://www.facebook.com/groups/chryslercrossfiretechrescue/permalink/1408006986382248
6) Check the AC blower motor testing the speed levels and the climate control directions as the blower motor and resistor go bad (they were on mine when I bought it a year ago) https://www.fcpeuro.com/products/mer...209342#fitment and Youtube showing the process
7) Check the antifreeze coolant tank at the tube entering the tank as this is known to break (mine was and I replaced with a new tank from FCPEuro https://www.fcpeuro.com/products/mer...mer-1705000349
8) Check the idiot lights that may be active with the car started and look for the low tire light. This is the TPMS sensors where the batteries have outlived their live span. You don't need this as it is an idiot light only. In the Files is the direction to disconnect the light (easy to do I did it when I bought my Crossfire) Youtube video on how to do this
9) The siren located under the front windshield cowling batteries corrode causing a battery drain. Either disconnect, remove or replace with new siren with a chirp. Here is a very good write up on accessing, disconnecting and removing or replacing the siren https://www.facebook.com/groups/202200410510109/permalink/1034931033903705
10) Clear all the drain holes especially the 2 located under the windshield cowling. They get plugged and there will be water on the floor
11) Check valve cover gaskets for leaking oil
12) Roadsters with an original top may have or encounter the rear window separation.
13) Coup interior headliner is known to separate
That's all I can think of at this time. These are very reliable sports cars where the MB M112 3.2L V-6 is a strong and motor as is the 722.6 5 speed auto transmission and the 6 speed manual.