Very good advice, didn't look at it that way. You're talking to a flat road runner for 30+ years, with just a single fell run under his belt at coniston.IMO the best method is to start with 2 pair and then causally add some when you see a pair on sale that you like. It's important not to simply by 4 pair of the same shoe, although some do this, but instead by different brand or style of shoe. Rotating helps with a lot of issues, but the primary benefit is you change the stress pattern on your foot every time you change shoes, which means you aren't only reinforcing a specific stress pattern which might be contributing to injury. Also, rotating 4-5 pair of shoes should make the shoes last at least 4-5x longer, more or less, so you're only spending more in the short term.
I need more fell running shops and event's to source these shoes, not much of that happening in East Anglia. A visit to epicentre in Ambleside last month was the closest I've been to all thing's fell running.