I have contacted my local Mercedes dealership, who are located next door to the Chrysler dealership after some too'ing and fro'ing the salesman has confirmed that a mercedes seat will work in the Crossfire. The cost is £380 +VAT, and they are launching a new range of seats in April (and new prices!!)

I have also looked on Ebay and a number of people are selling seats with transponders fitted that they claim were purchased from a mercedes dealership but will work in a number of cars (a mini was one I enquired about)

Personally I won't buy a second hand seat, in case of accident damage but it was useful to make those contacts on Ebay and is an option when I need to sell the one I have ordered from Mercedes. I get some of the money back when my little chap is 4years old.
Mercedes do baby carrier seats and toddler seats so you shouldn't have a problem


Good Luck!! Happy driving ..

PS also got my chap a sun hat in case the weather improves and we can have the roof down