We have an existing external system whose API documentation mention that the client system should support XML-RPC specification. To consume the services on this external system, can we directly do a HTTP Request with XML as body? Is it supported, if yes, how complex it can get?
Apex does not directly support XML-RPC, but instead, its successor, SOAP. If possible, use SOAP to call Salesforce. If you can only use XML-RPC, you'll have to manually parse the request yourself to implement the appropriate logic. This can range from trivial to incredibly complicated. If possible, seek alternative options.