Web Services - Webservice/asmx/ashx/whatever Programming??

I need to build a proxy (maybe a bad description) that receives an XML file from a 3rd party, saves it, sends it on to another 3rd party, gets the response back and passes that back to the original 3rd party. Let's call that entire process a "unit".Should I use a webservice? A Generic Handler? Something else?

I might have to do 20 "units" per second, but I know that each "unit" may span 30 seconds to a minute each, so really, I mean that I need to be able to have 1200 of these "units" running at the same time, in all varying stages of the process that I described above.As far as the file saving goes, I eventually want to put this into a database, but I would imagine that writing the file is quicker than actually saving the data into a database, so I'll just have another process that isn't nearly as time critical as this grab the files and insert them into the DB at its own convenience.

The "app" will only consist of 1 page and it will be running under SSL. This will likely be the only thing on this server at any given time to ensure that this little process is not a bottleneck.What in .Net would be a good (fast and scalable) way to go about this? I don't have any effective limit on what I would need as far as hardware goes -- so I can get a screaming machine if it would guarantee no bottlenecks.