My HTTP Endpoint responsible for downloading the file at the end of the flow is erroring out. It keeps trying to communicate with http://:80/ instead of the URL
I can't spot the error: your flow looks good (except a problem I'll detail below) so, alternatively to using an http:outbound-endpoint, which can be finicky at times, you could use the http:rest-service-component.
Your next problem will be in the outputPattern of the file endpoint: remember at this point message.payload will be the response from the HTTP endpoint not the (partial) URL anymore. You need to pre-compute the file name and store it in a flow variable, then use it.
This would give: