Unless you're buying a shed, that MultiMac car system will likely be much cheaper than the cost to change into an MPV. That said, I do know where you're coming from as spending the value of your current low value car on a custom child seat solution only for your car to go pop the day after... Well it's understandable you'd be wary.
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Good luck mate, it'd be interesting to hear how you get on with solving it.
While it is true that the op could be wary for that reason, once the MultiMac is bought, it is bought, and can go with him to whatever the next car might be. Without limiting the choices of what that next car could be. But yes, having a child seating system worth 2-3x the value of the car it's going in might be a stretch.
I am also curious as to how the OP solves it. My wife is talking about a third soon. This problem doesn't really apply to us because we got an appropriate vehicle the last time round, otherwise we would have been in a very similar situation.