You've done this more times than me, but I'll give you a call later to impart some learnings... but for the benefit of the forums, last time I did this it worked out pretty well. The CEO ended up calling it "the jewel of the company"... but be warned, do it too well and your space will be the source of constant tours. Pretty much anytime anyone of any level of import (mainly retail buyers) their was a substantial tour of the design center, that was accompanied by an extensive dog and pony show (get the 3d printer running, put out a bunch of sketches and mock ups on the conference table, etc) ... not necessarily a bad thing, actually a good thing that we were valued, but just something to plan for.
I broke the space down into these key areas:
Lots of shelves of books, material swatches, historical product, product references, a pin up area showing the latest design language system documentation, and a sofa and a few comfy chairs. It had 3 functions, casual meetings and work, hold all of the color and material references, cool looking show piece, you actually walked through this to get into the design center proper, it was a small space but pleasant.... eventually I also got a ping pong table and full forza racing rig that lived on the periphery.
Giant table that sat 12 easily surrounded by pin up space and a giant projection screen. It was nice for design to have it's own space to meet and where we could leave the latest on every project pinned up. This functioned more like a war room. It was cool though so other groups always wanted to have meetings in there. I had to institute a policy that we would share but I could boot them as needed. We also ringed this area with lots of shelves that could hold 3d prints, final models, and pre production samples... this was more for show but I think people felt good when one of their projects got to the point where it went into the show case area.... just shelves.
Not really a shop, just a spot where you could make stuff. Consumer level 3d printer, plotter, I would have gotten one of those new consumer laser cutters, you could do some light foam sculpting, print a render out on a plotter full scale and wrap the foam with it. That kind of thing. I kept it super light and convinced the engineers to buy and maintain an Objet and we would go out of house to one of the big rapid prototype shops 12-15 times a year for anything hyper realistic. The engineering dept also had a real prototype shop so we would use that resource as much as we could.
Goes without saying, but you need heads down space for people to crank.
Furnishings. I think the budget on furniture can swing widely. I spent all the big coin on great chairs. You got to sit in that thing all day, then cheated out on Ikea desks and shelves... because they are fine.
I commandeerde an unused industrial space in the building so the whole thing had the right level of messy, casual, start up feel. The zones I listed above were pretty soft, defined by shelving.
I started with a smaller space, the company liked it, I then got us a bigger space as we expanded the team... then eventually I got to redesign the building and pick all new furniture. It was fun, but I also became the repository of all complaints about the building and as you know, no design pleases everyone....
This was the first smaller space I designed (each square on the grid is a foot)
This is the second larger space. This required a much bigger furniture plan and instructions for the GC as we moved some walls and added lighting etc...
And then this is me eventually planning out the larger building... it started to get a little crazy working out multiple floors and communicating with GCs. Eventually the Ikea furniture wasn't cutting it and I brought in a legit contract furniture partner to help me plan out finance, sales, and op's work areas.
EDIT: I forgot, after this phase I added on a photo/video studio to shoot, a sound mixing/recording room (very light, just prosumer gear to record decent VO's and music for product videos), built out a showpiece listening room with a historical product display... so do this well and you will probably be doing more of it