I'll just add my two cents based on my experience building an ITAM/SAM framework from scratch.
You have few sites, low problem on interconnectivity (WAN) I hope and I assume you have SCCM agents running from more than one server, so it is the SCCM data collection that has impact on the network not the interaction with the SAM system. (be careful if you activate Usage Metering.)
Other agents can be smarter and have lower impact with more details, but it always will depend on the frequency you need to update your data. How frequently are your PCs changing configuration?
Big server base, what kind of software is there? Is SCCM able to recognize Oracle installations, IBM PVU (sub capacity), true MS SQL installations? OK, otherwise you will have to work on both the SAM tool agent data and the SW installation process (packaging and standard rules)
Do you have SW that uses Flexnet license distribution SW (e.g. Autodesk, Minitab)?
In that case remember that Flexera is the owner of that technology and has specific products to gather usage information and integrate it in the SAM Suite.
How many SW titles/versions do you have on PCs?
Are they "closed" PCs?
The critical point in any SAM System is the quality of the Software Recognition module that is based on smart algorithms and on a strong and timely updated database of references about how to identify the Application "reading" information from data sources.
How frequently the vendor is able to update the reference database and how fast they can answer an automated request to identify an unknown SW found in your environment is a really important factor to evaluate.
Good to have is also a library of pre-defined license models to quickly load your entitlements without having to reinvent the wheel.
I'll not be worried about integration. In some way everyone gives you the opportunity to connect to the SCCM (or ILMT, or HP DDMI) databases to get data from. Flexera does it out of the box and if it is not enough you can always write scripts to interface other source (like CSV files...)
Ideally this tool would help to integrate a self-service service request.
Do you already have the self-service tool (or any ITSM tool)?
Otehrwise Flexera has the self-service tool that can be integrated with their SAM Suite. and the Installation SW + license distribution if required.
Is this something you can be interested in?
As said above by others, the first step is to understand what are your information sources, NOT only installation/usage data, but detailed Purchase orders, HR, License documents, user's requests, and so on.
You'll need to understand how this data will be made available to your system and design really strong processes to catch changes, planned or unplanned and to ensure the data that is coming is reliable. This will not be provided by any SAM SW but by your IT, procurement, and HR processes that you will interface.
You have 1500 servers. Do you have a consistent CMDB in place? Do you know relationships between CIs? Is this being maintained effectively?
If affirmative, then you can plan to integrate it into SAM and populate one each other. (why to duplicate tools and processes?). Maybe you can realize that you only need the Software Recognition module that can be sold as a service by some SAM vendor or by other SAM companies.