There are lots of equations to deal with when buying a used car. Availability of Auto Trader divided by newsagents open at 7am on a Friday, multiplied by cups of tea in a cafe, for instance.

Or phone calls to shifty blokes who will only agree to meet you in a car park divided by how many examples of the car you're after are available on that day. That one never works in your favour.

But it's the technical equations that really get me going: equipment levels expressed as a fraction of the cost when new, engine size plus bhp plus fancy interior kit or, my favourite, list price divided by massive depreciation.

And there's little that depreciates as much as, or is as well equipped as, a top-end BMW, which is where the M535i comes in. Once this was the quickest car BMW made (140mph, 0-60mph in 7.4sec), and cost £17,950 new.

This model came before the M5 and the first M535i was the original M-car. It too was fast, agile and lethal in the wet, but most have either been stuffed into a ditch or artfully bent around a tree. The second-generation cars are more plentiful and easier to live with. And if you could work out a way of applying those daft equations to them, you would get impressive results.

The raw material consists of a massively characterful and hugely durable 3.5-litre straight six, with 220bhp and 220lb ft, Bilstein gas dampers, a limited slip diff, leather interior with Recaros, electric mirrors and even heated door locks.

I couldn't resist. Mine was white and looked like it had been stolen from a sink estate. Once it had a turbo. That had long gone, but 143,000 miles had loosened the engine to the point where a turbo wasn't necessary. The wings were shot through with rust, as was most of the underbody, and the interior resembled the aftermath of a plane crash. It cost £500 and not once did I regret handing over the cash.

It pulled like a bomber from 1000rpm, and it was a manual. Always get the manual. Yes, the shift is obstructive and the clutch heavy, but the enjoyment experienced from opening the throttle and downshifting from third to second, accompanied by the straight six's deep resonance, is worth it. Compared to a modern sports saloon it feels light and immediate, with its barely assisted steering and taut damping.

Convinced? If not, then let's talk money. I wouldn't suggest going as low as £500, but £2000 will get you a really good one. M535is will never be worth much because they will always live in the shadow of the M5, so don't expect one to become an investment.

Besides, it will have rusted away into nothingness long before that happens. But that means this car will always be subject to lack of recognition divided by immense ability, and that equals a bargain.