XML Secure Data Service

[Wizfrog]

Hello,

I'm trying to figure out what would be the best technology to work
with in terms of network connection and services to do the following:

- I want to build a Window based application for internal use only.
- The application needs to access data 'somewhere' in a secure way,
with different access levels / credentials.
- The application needs to send some other data to the central place
as well.
- The application needs to work offline using the data it will have
downloaded.
- The Data will be XML so it can easily be converted into Word or PDF
documents and EXCEL reports.
- The Data is edited only by selected users, most users will only use
the data in read only, but send other type of data back to track how
it was used.

In terms of network access, I am wondering what would be best; public
web server would require pretty good login security to avoid being
hacked.

I'm thinking about using access to an intranet through VPN, with a
login process checking credential and filtering the DB with XSL
through something like a CGI or PHP script, and send over only the
parts of the DB the user needs, and store it on the local machine for
offline access.

For the interface, I'm using C#.

Any better idea? or suggestion?

I am also thinking about mobile phone access: I have no idea if cell
phones are able to use VPN access. Anybody know?

Thanks for your suggestions.