Hi
In my opinion , the first step would be to sit with 3rd Party technical team (in your case JMS receiver) and decide on what fields they need .
Generally start of any development is creation of functional specs(gives functional overview on what need to be achived by the interface) and technical specs(underlines the technical touch points, Mapping sheet).
So you will have the receiver structure , then back to your questions on sender side you can use proxy or idoc.
Proxy- Using proxy , you will have flexibility to use only those fields which are required by target system mapping can be made simpler ,infact you can use same structure as target requirement (make the structure generic).you can find a walkthrough
IDOC - using idoc will give you advantage to use standard functionality , like in your case MATMAS idoc can be used.IDOC structures are not readable , so you would need to refer the functinal names by checking the tehcnical documentation of IDOC and create a mapping sheet.
Regards
Srinivas