It all boils down to your budget. The more you can afford should give you a better car, but carry out all the checks first. Ideally have a Crossfire owner accompany you when you go to look.
One point to note is to get the lower tax bracket (£300+) compared to the higher tax (£500+) by getting a sub 2006 (March I think) or to be sure get a 55 plate one.
You only have to look on Ebay to see the average price of the Crossfires. Lots for £3k and under.
Garage, and low mileage examples, tend to obviously cost more. £4k plus.
From a garage is good for the warranty (usually at least 3 months- well for the Mercedes parts not the Chrysler parts!) but a low mileage, loved, private example would be my choice. Just make sure you check
all the history and the bullet points raised on this site for things to look for.
If you go for a high miler then make sure the service history is up to date: spark plugs and auto transmission fluid changes at the 100k miles point, to mention a couple of things.
If you can wield a spanner then ownership can be fun and helps to keep the cost down.
Good luck