Yes, you are right. You will need to create your own GeometryService.
Here some documentation that might be useful: http://help.arcgis.com/en/arcgisserver/10.0/help/arcgis_server_dotnet_help/index.html#//009300000027...
Just an idea: If you just need to do buffer on a point, a little bit of trigonometry using Cosine and Sine would get you going all on the client side, so you don't need a service