If you would buy Ambassador, make sure to buy it upon check in at the hotel you have this 3 week stay. Ask them upfront what they are willing to throw in (good upgrade) when buying it with them. This might sweeten the deal, and make it worth it even for just this single stay.
Careful with that advice. I don't know how it is these days, but in the past you wouldn't have been eligible to earn IHG Rewards Club points for the entire (30-night) stay when you bought Ambassador status at check-in... and that's missing out on a whole lotta points!!! I therefor suggest doing it prior to your stay.
Also, I did a random check comparing both regular rates to the corporate clinic rate... on al 5 nights checked the roomrate was identical, with the exception of the corporate identification number added. So instead of you getting a discount with this rate, I rather think that the clinic is receiving a commission on your hotelstay. If this is the case, you're better of booking BAR through a cashback site, for instance there's currently 6% cashback at
www.mrrebates.com?refid=1129557, I usually use
https://www.shoop.de/invite/fvikdr1s5v/ which currently gives 12% cashback on InterContinental Hotels, but it's a -very efficient- German site which might be challenging for most people whom don't read or write German.