Network Manager, Fixed Asset Manager, etc

When it comes to add-ons, I think you should feel the need, first. Over the years I've spent too much money on things I thought I would need, before I needed them, and needed to buy an upgrade before the first even paid for itself.

If you handle a lot of new accounts, requiring a lot of new entering of assets, then the fixed asset manager may save you a lot of time. In ProSeries, once you have entered the first asset list, the assets will carry forward every year, and disposing of some and adding a few new ones is easy. In fact, I have several ProSeries files that are not for use doing taxes since the income taxes are done on other systems. But I use ProSeries to give me the annual depreciation and lifetime depreciation listings, which I add to the accounting record and post from. It is far easier than the old paper and calculator ledgers, so prone to a variety of errors throughout the years. 

When it comes to a Document Management System, I am more skeptical about using it - I've been using another version since before Intuit came up with one, and it handles multiple functions for me. Intuit DMS is a good program although I see complaints often - a decent training interval will handle that. I hear even more complaints about non Intuit DMS's which are very expensive - and the fundamental issues are the same. If you feel the need to implement a DMS system then Intuit's is good.

That said - the foundation of DMS is computer housekeeping - creating a filing system for all the documentation you want to keep, and a system for getting it into the computer, and a system for backing it up, and a system for requiring that everybody always do it the same way so the information can be identified and used by others. From my perspective the last reason is the first reason to implement a formal system - to make everyone do it.