If you were starting an Architectural knowledge base from scratch, what are the key deliverables you'd produce?  eg. I imagine System Context diagrams of the current state would make the list.   Would you use a dedicated Architecture tool right away or start to build out the practice with generic tools like Visio first to clarify your requirements before purchasing?

I would encourage strong consideration of buying and implementing an architecture tool to get started as we found the structure elements that a tool provides a single place to reference and 'see' forward progress was valuable vs. perhaps just a sharepoint site of diagram documents.  

I would take a step back and determine where the architecture knowledge base is going to bring most value - what are the pressing business challenges? This will help identify the number, type and complexity of the scratch artefacts needed. I would then start with the tools you already have - Excel, Visio, SharePoint etc can go a long way, both to address the business challenges and to better understand your requirements before committing to what can be expensive tools.

Using tools heavily depends on the use case, that you’re having to solve. 
In times of struggling busines, EA is o often asked to support in cost saving initiatives. That is usually done by establishing an Application Portfolio Management (APM) with the wish to give guidance on future directed decisions on the portfolios. You need to rate applications on their business contribution and on their technical maturity. Then you can apply Gartners TIME methodology to (T)olerate (I)nvest, (M)odernize or (E)leminate applications. 
This can be done with Excel, SharePoint or EA tools. If you decide to (M)odernize, you can use the Amazon 6R framework to decide on replacement, repurchasing, replatforming etc. 
Often, the company wants to better manage technologies or drive for cloud transformation. Then you need to collect e.g the Software or cloud products and analyse redundancies in the capabilities they provide or hosting locations and technologies and drive for relocation workloads into the cloud. 
The tooling is secondary. The task for you as enterprise architect is to align the use case with the business and IT stakeholders and define the data needed to drive the use case with KPIs. The data collection should be user friendly and the value of the exercise should be clear.

I would not use an architecture tool right away. They are expensive and you are already attaching a big ROI to your team and practice before you even start along with the requirements. I would start with Visio (or I prefer LucidCharts) and spreadsheets. This will allow you to use tooling you most likely already have or can get much cheaper to show what architecture can provide to the organization.

As far as artifacts to start with (assuming we are talking about an overall architecture practices and not infrastructure):

- System integration diagrams that show each critical system and how they interact with other systems. This should include data flow and the type of data itself (classifications if available)
- Create an Application Portfolio. This is critical to do along side the diagram creation as it provides insights into the solutions you have and the capabilities they provide the organization.
- A full end to end Enterprise Landscape diagram that shows all the critical systems and how they interact to form your enterprise and serve your customers.

Hope this helps!