I have not managed so much the Drupal system, I only install it once and I tested it very little. However the last year I had to develop a SUSHI client in php and I did tests with various tools for managing Web Services, as the PHP SOAP extension, the PEAR SOAP-Package and a SOAP toolkit for PHP Called NuSOAP. I guess that Drupal has to handle some of these tools, unless they have done their own implementation, if you would tell me the exact version of Drupal is in which you want to do this, I could review it to know how Drupal handle it, because it could be different from a version to other.
My experience with those tools I mentioned earlier was not very good, because my client needed to obtain reports from several SUSHI servers and none of them succeed in making it work with all servers that I required at that time, so it also depend on what you need, if you intend to connect to a single server or a few, maybe you can use any of them. However, if Drupal has its own implementation, perhaps it can be used to connect to multiple servers, is needed to be revised.
Either way I would recommend that you try to rely as little as possible of Drupal, as this could be an obstacle to that in the future you can upgrade to new versions as they arise, firstly because it could change its Framework, as in the case of Joomla when they upgrade from 1.0.x to 1.5.x and also because later you maybe want to use another system instead of Drupal, but hey that's your choice.
If you're interested, in the next issue of the NISO’s ISQ magazine, there will be an article where I talk about my experience developing the SUSHI client I mentioned above, there will be some things could be helpful when you start to develop your SUSHI client.
Regards
KMLM List