The real question is, does cruising at 90 really put much strain on the car? I suspect not to be honest. If it's being treated like a rally cross on the other hand...
Cruising at 90mph won't. Flooring the car from cold will. My dad worked some time as a sales exec, and even now as an accounts manager he's forced to do a LOT of mileage. The deal he has, is that he keeps the car for 4 years, then trades it in for something else. He doesn't pay petrol, tax, insurance, maintenance...But he really does drive it like he stole it. I wouldn't be surprised if that BMW was a repair/write off job, so if you do decide to buy it, make sure you get it electronically checked.
Weird as it sounds, i'd rather go out and buy a Roadster than most saloon cars, certainly don't need the extra space.. and yet the 5 series appeals some how
If you want my opinion (which you probably don't), here's a short-list of cars I'd be looking at in the ?8k region.
Stupid Options
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1975 Alfa 1600GT - ?6995
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1987 Audi Quattro 10V - ?7595
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E30 BMW M3 - ?6000
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FD3S Mazda RX7 - ?4795
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Track-Prepped Impreza Type RA - ?8250
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1990 Formula Renault car - ?5995
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'78 Corvette - ?8750
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'74 Ford Escort - ?7000
Slightly more intelligent options
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'02 Honda S2000 - ?7695
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Cat D repaired S2 Lotus Elise - ?8500
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Honda Accord Type-R - ?5500
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Nissan Skyline GTR BNR33 - ?7995
Best option
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'98-spec Honda Integra Type-R - ?7495
Now, obviously a lot of these are daft, and you probably couldn't insure most, let alone afford to run them. But with the current economic situation, it really is a buyers market. ?8k at the moment for a car is a LOT of money, and it would be a shame to see you waste it on a shagged rep-mobile, when you could potentially buy the car of your dreams. IMO, for the money, a '98 spec Integra will do everything you could ever want it to do. It'll return over 30MPG in peaceful comfort, then it'll devour most things on track, and it will get you home again without ever experiencing a problem. The one I've listed there is fetching a lot of money due to the modifications and the history, but you can pick one up for around ?5k, but if you go that route, make sure you go for a 98+ spec car. Until recently, me and my partner had been talking of selling both our cars to buy one (though it's no longer possible as I'm joining the RAF), but my best friend has one, and it really is an incredible car.
Anyway, hope that helps, and good luck with your search.
Pistonheads is a great site to find the more unusual cars for sale.