Hi Michael,
What you mentioned above is possible and it can be done in many different ways.
Easy Method
You can use the Retrieve value from table action to retrieve a sum to show on the screen, in your case a sum of invoices based on sales rep and date range. As for assigning a budget you could create a field in the Sales Rep table where you can store a budget.
Not so easy Method (But really awesome looking)
You can create a chart, similar to the Sales Centre Dashboard. This blog post here goes over creating a chart in Method, the same concept found in the blog entry can be used for sales rep instead of customer.
Another option is to make a report in Method that can be generated as a pdf and displayed or printed.
Pip pip cheerio
-Michael