I think trevs covered some really good points there. If your buying private I personally think alot can be read into they way the seller interacts with you. You kinda a get a feel whether he's been open and honest or he's hiding something.
I got a little 2002 Corsa 1.4 tie me over for £500 but, the old guy had receipts for £400 of recent work. I wasn't sure if they were gen as they were kinda scribbled hand written with a stamp so I rang the garage just to make sure they did the work and he confirmed it.
The best way if your unsure is to draw up a list of things to check "what to look out for":-
or take someone who knows cars to give it the basic once over.
To be honest, in that £500-£1000 bracket it's a gamble. In my time I've had cars in the 5k bracket go t*ts up within weeks of ownership. If you do your checks and something comes up and your not sure simply walk away, 2nd hand cars are 10 a penny there'll always be another.
Good luck I hope you get sorted