Hi Dave,
Congrats! You have a challenging but very rewarding role to play in your organization. A BA Center of Excellence can (and in my opinion, should) leverage whatever is already working well in the company, and seek to achieve excellence through continuous improvement. So treat this like any process improvement project that a BA might tackle.
Assess the current state of BA practices. Compile assets that exist - templates, examples of deliverables, tools, techniques. Then do a SWOT (strengths, weaknesses, opportunities, threats) analysis of current BA practices. That will tell you which assets are working (and should be kept), which are not (discard or improve), and gaps where you don't have the assets you need.
Other BAs in the company are now your clients. You would not tackle a process improvement project within your finance department without engaging finance staff, would you? (I'm hoping not! :) ). You need to work with your management team to identify BAs who can represent the organization, and get approval for some of their time to participate.
Change management will be a big part of your job. BAs (and other IT professionals) tend to be most resistant to change. After all, we're dedicated to delivering business value and believe we're doing the best job we can - so why should we change? (tongue-in-cheek...). How well you achieve your mission will be highly dependent on how well you engage your BA community.
For your presentation, I would suggest treating it as a kick-off. Introduce yourself and your team. Describe what a BA Center of Excellence is, and why your organization needs one. Describe your goals and explain how you plan to move forward. And most importantly, describe how you plan to engage the company's BA community, and provide some channels for people to get more information & give input. This will set the stage for a collaborative partnership, rather than the 'commander' approach that you already know won't work so well.
Good luck. and come back to this forum anytime you need. We're always here to help.
Sandy