Here’s the easiest way to convince them to not use the Broker–it won’t work. 
Broker holds messages in their entirety in memory. For your situation, a 200MB file might work, but 1GB almost certainly will bring the Broker down. Broker is designed to handle lots of relatively small documents, not huge batch files. One of the primary reasons message brokers exist is to allow the shift from batch processing to near real-time processing. That means that business events (new customer record created, order received, invoice issued, etc.) get published right away individually, not held for batch processing in big groups later.
The appropriate mechanism to transfer between IS instances would be to stream the file and process the file as a stream on both sides.
"…and has guarenteed delivery, supports asynch, decoupled integrations and so on… "
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Guaranteed delivery – IS has GD facilities too. Review the services in the remote folder in WmPublic. Remember too that Broker GD is between the adapters and Broker only. Once “your code” has it, all bets are off (you have to do the GD work).
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Async – Without Broker, there is a bit of work needed to do IS-side queuing. The IS GD facilities can provide this to a degree (it retries for some time before giving up–you can configure this). You can use TN as well, though it uses the same sort of retry-a-configured-number-of-times approach.
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Decoupled integrations – Ooo boy you pushed my wrong button on this one!
IS provides decoupled integrations too. The use of any intermediary between two apps provides decoupling. The document producing app has no idea who/what/where the receiving app(s) are. There are a couple of threads on this topic that you may find helpful.
[url=“wmusers.com”]wmusers.com
[url=“wmusers.com”]wmusers.com (unfortunately this thread didn’t get much action)
[url=“wmusers.com”]wmusers.com
[url=“wmusers.com”]wmusers.com
HTH
#webMethods#Integration-Server-and-ESB#webMethods-Architecture#webMethods-General