Hi there!
The Britax B-Motion 4 is not compatible with any conventional additional seat. It is a mono stroller suitable for one child only. It allows you to create a stroller system, and that's about it. Personally, I would not add much to the chassis, even if it was allowed- the chassis was not built to handle such a lot of weight. An additional side seat - the Buggypod would be the maximum of what this stroller can handle – and it'll be relatively ok. Once your first child is able to sit and remain seated independently, the Buggypod could be a good option. Of course, adding an additional seat or a sibling board will affect the maneuverability of the stroller slightly, but it is not something you wouldn't be able to deal with.
You are right, the Britax B-Ready or the Baby Jogger City Select Lux are more expensive models, but that is the general expectation from such a type. Usually, the full-sized double strollers come with a more significant price tag - these strollers have to be sturdy, higher quality, and durable enough to accommodate two children - therefore, it is worth to pay more for them. Alternatively, you can save a lot of money by getting a second-hand stroller. It won't be a brand new and shiny, but still the high quality, practical tandem stroller you need. The B-Ready should be compatible with a carrycot and a car seat.
The Britax B-Dual is already an older and not too great model. From a practical point of view, it is not something I would consider. Nowadays, you can only get a second-hand B-Dual.