So what's the answer to such an operation? A one horse outfit doesn't work. Stby a/c are too expensive.
I always recall a conversation I had with Andy Janes (of Emerald notoriety) one evening and Andy explaining to me that his fleet of budgies had pretty much hit zero value so there was no hull depreciation to take in to account, the aircraft were paid for so there were no mortgages to pay and, pretty much, besides any parking fees, insurances and in-house maintenance costs any standby aircraft cost was, pretty much, bogger all.
So perhaps the answer is to operate an aircraft type that has become surplus, many years ago the MD80/90's became surplus, thus cheap, and a number of operators took advantage of that, likewise the Fokker jets became surplus and cheap and a number of operators took advantage of that also, I've read on this forum, to the effect, that one can't give A340's away, I don't know the going rate for the B757/B767 family now that the B787 has become all the rave, etc.
As for reliability, in one of my previous lives I worked for a geriatric DC8-62 cargo operator in Europe, hydraulic pumps were the favourites to fail, firstly they were buying the cheapest and nastiest reconditioned hydraulic pumps from Florida that, by the time one arrived in Europe, a 24 hour delay had already been incurred but it was common knowledge that if one hydraulic pump failed then the other three pumps had been allowed to run dry and they too would fail soon afterwards but would that operator ever buy more than one pump at a time ... no!
So, perhaps the answer is to operate a surplus aircraft type and, whatever the type, have a good spares back-up.