Not sure how much extra power you would be satisfied with, but the M113 5.0L V8 engine could be a potential swap. I would not be surprised if it bolted in directly, motor mounts, headers, hoses, lines, and all, since it is essentially the same engine but with two more cylinders. Plus it can be found for quite cheap.
The AMG engines such as the C32 are great fun until you need to rebuild or repair them and discover the costs of the replacement parts.
I have been curious how involved it would be to get the M113 running in a C320 or any other car originally equipped with the M112. Most of the connectors on the engine harness are probably already the same. It could be a huge pain or it could be as simple as swapping over the engine harness and ECU from the donor vehicle.