I take it you know that the frame needs a split in the rear triangle, so you’ll need to budget for a new frame, or a modification to your existing one, on top of your £220 ?
If you’re using an existing frame, you’ll find the available gear choices may be very limited due to your chainstay length. I picked my gear ratios, then got my frame built to suit.
Two of my belts broke due to damage caused by getting small stones trapped between the belt and pulleys. One punched a clearly visible hole in the belt, which carried on for several weeks before suddenly failing.
The other broke due to shear volume of packed grass and mud getting wedged between the belt and pulleys. If I’d stopped to clean it out properly, it would probably have been OK. I was racing 24 hour solo and tried to keep going.
Ask anyone what the conditions were like at SITS last year. Sure, I wrecked a £70 belt, but plenty of other people wrecked £70 derailleurs.
The difference is, you can buy a derailleur anywhere the next day.