As an ERP there is a ton of great functionality. To have all of our core functions in one place (time entry, expense reimbursement, budgeting, billng, and GL) greatly simplifies management of a professional services firm.
The reporting functionality that comes with the standard system is not so strong. There also seems to be a continual shortage of individuals at Deltek to provide support.
Work as closely as you can with the standard setup. Customizations in both reporting or structure make utilizing the system more challenging at the time of updates.
You should plan in your budgets for at least one upgrade a year, with everyone other year one being a more significant one where you would likely incur $15k to $20k of expense and then every 10 years an upgrade that is more like a new system implementation.
On licenses, you should do a growth analysis of users. When we acquired the software we bought just the number of licenses we needed. In this number we got a good bulk discount. Then in the future we started adding 10 to 20 at a time. The bulk discount is based on each individual purchase so we didn't receive any discounts when we added licenses and are paying significantly more for our new licenses which are now about equal to 50% of the original purchase. There is no way to start that over.
We have created a very efficient online billing process that allows individuals responsible for billing to have access to real time information. The ease of which we can move from client based data to GL data is also tremendous.