Hey, Steph,
Double prams will always be a more expensive product, and you think right of getting one second hand. The Wave, however, I am not sure it that is a good idea, as the seats are not too large, and it might be a very tight fit for a 27 months old. I suggest trying it out before buying. Other than that, the Maxi-Cosi adpaters for the Wave should fit a Cybex car seat with no problem, however with the larger Cloud Z, it may be, again, quite tight to click on both parts on the frame.
YOu can also check a used Bugaboo Donkey, again, with Maxi-Cosi adapters, the Cloud Z should be fine, and the maneuverability will be considerably better that a Wave's. Also worth checking out is the ABC Design Zoom which is longer but better in terms of maneuverability as well as spaciousness, too. Again, you will need Maxi-Cosi adapters to fit your Cloud Z. And lastly, I will mention the Baby Jogger City Select Lux which, again, is smaller in the seat area, but fits a Cybex with Maxi-Cosi/Cybex adaptors, plus offers a bench seat accessory if your older one won't fit in the seat anymore.