1.) DotNetInvoice will definitely run on Windows Vista. You'll need to have IIS installed (which may not be available on the Home version). You'll also need to install SQL Server or SQL Server Express on that machine or have access to another machine with SQL Server.
2.) We offer custom development for DotNetInvoice at $125 / hour. When you are interested in a feature that has been frequently requested and we are considering for our next version, we offer a large discount on our rate. In our email response to you, we detailed the easiest implementation of this feature.
3.) We currently have many customers with larger customer databases that are not experiencing any issues.
If you have any additional questions, please feel free to continue this thread or email us at support@dotnetinvoice.com.
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Jeremy
DotNetInvoice Support