Value the potential purchase on the revenue VRT calculator. Values on the web are normally inflated.
Go in, ask salesman to show you around car. Take a (long) test drive. Get the car checked over fully. Examine service history. Establish any faults.
Be friendly but firm.
If you are still interested ask him how much for cash. Say something like "that's better but I've seen 2 others with lower mileage cheaper". Tell him what you want to pay for the car less say €500 (for negotiation). Take control. Point out the cost of any jobs that need doing to the car.
If he's miles off just say "We'll never reach a deal like this" and in a friendly and polite way, just walk away.
If he calls you back you are quids in.
Ask for extras - service, valet, new tyres (if needed), mudflaps, floormats, some road tax, etc.
Knowing exactly what the car is worth day 1 is the key.