Do you want to and have the bandwidth to develop, monitor, and maintain your own custom solution? If you don't mind doing that, then going the route of a custom .net-based, MSMQ/WCF solution might work well.
BizTalk will also cover all of the requirements you have listed. There is a learning curve but it is certainly not insurmountable. The initial ramp-up may be lengthier than would a custom-code solution, but there are considerable benefits, particularly the benefit of having all your requirements reliably met:
- secure
- transactional
- reliable (messages aren't lost)
- highly available (24/7)
- failover
- adapter architecture (includes polling adapters)
- transformations
- working with external web services
- returning correlated responses back to the source system (i.e., orchestrating the end-to-end process)
- use a broker (you specifically listed this, and BizTalk is a broker; custom MSMQ and WCF means using no broker)
If BizTalk needs to poll the POS system, then you do not need to worry about using MSMQ. BizTalk can handle transferring messages reliably (they're persisted to SQL Server, while MSMQ persists messages to disk).
Note too that the only way to make MSMQ highly available is to cluster it. So either way you'll need to cluster something.
A BizTalk solution will be easier to maintain over time, particularly if you just want to update your transformations. With versioning you can do so in a way that doesn't require downtime. It'll be tough to update a custom solution without downtime.
Some people have had difficulty in the past with monitoring BizTalk for failed messages, but I have found it to be easier, especially with a tool like SCOM or BizTalk 360, than trying to monitor message queues, which often requires even more custom work to monitor. Just make sure to include monitoring in your cost estimates for the life of your solution.
If you do need auditing, then BizTalk also has you covered. MSMQ Journaling will keep a copy of each message for you, but without significant transaction details and with no out-of-the-box way to search through or archive the data.
Building your own .NET client code to work with a Java web service will likely take a good bit of work regardless of which way you go. With BizTalk that means running a wizard against the endpoint or against the WSDL. With WCF it means doing everything by hand or with the assistance of the svcutil tool.