- where and how you host your data
- what api your partners provide or need
- how to process / generate / query / store XML data
As you are building an internet service you could think about using a platform as a service in the cloud. Then, you don't have to take care for
- scaling as you grow
- automatic failover
- disaster recovery
Also, as you are building a XML based service you should use a programming language that can deal with it best. Most people are tempted to use a programming language like PHP, ruby, Java, or ".NET" to process XML and store the XML data in a SQL database. If you don't have plentiful of time and money to waste: don't do that because you have too much overhead for design, mappings, parsing and marshalling of XML data. It is much more efficient to use XQuery for example (From my experience I would say that you can safe around 80% on average for code, design, testing, maintenance ... ).
Check out for example: http://www.28msec.com. They provide a database in the cloud where you can store XML natively out of the box. Plus, you can implement complete websites or REST apis only.