Benwrx

I've just got rid of my cav gsi after many years of service and am going to look at my first scooby this week. I havn't got much of a clue about common faults or other signs of a dodgy car. I know about running optimax and getting a knock sensor but am I best getting the sensor fitted soon as i get the car or could it wait a month as i'd rather throw money at getting a decent one to start with! I'm looking at one from about L-P reg for about £4k, are there decent ones at this price range? Any info would be gratefuly received!
Cheers, Ben!