Dave I have written some software using the SOAP interface so I can offer a bit of insight.
You can skip the checksum checking. Every system has a "Last Updated" field that you can check instead. The new SOAP interface is the way to go, the old web api is going offline at the end of July. It is open to anyone, all you have to do is click on "Account", then API, then click on Apply For App Key. Theres no extra cost to it but you have to be a premium subscriber to use the interface.
You should check the wiki SOAP page for more information. There is the WSDL which well tell you how to interface with the service. I don't think there is a way around this without writing your own app. I would suggest you download the free version of
VB. Net (2003) from Microsoft. Then get the WSDL compiler, which can download the wsdl straight from this site and generate the code for you to interface with the service.
If you want this to syncronize with your scanner, you will ofcourse need to write code to upload to the scanner as well. Since you want granular control, your app with need to download from the scanner first, or have provisions for user editing of saved files. If you have never written SOAP code or scanner interface code then you will have a great learning experience as well but if could be a lot of fun. Best of luck.