froob: Actually, if you do want a low budget option, I have a couple of Vodafone wifi extenders which might do the trick. I know that will look hypocritical, after literally telling you not to go for an extender in the post above... but they are extenders in name only.
Thanks for the kind offer, but I think I'll pass. It looks similar to what I've attempted to achieve with my current setup. Which has us in a state where we have to manually flip between wireless access points when perf degrades on one of them.