Are there cars out there now which are real dogs or is it like bikes where it's actually hard to buy a "bad" one?
Yes. There are real dogs out there. In particular cars with non-obvious faults, or even deliberately hidden faults.
My advice would be:
1)Avoid petrol Nissans of that age for the potential, quite common (and very expensive - like £1000+ to fix) timing chain issue.
2)If you feel that the person selling the car is dodgy, then leave - it might well be a complete dog that they've tried to cover up faults in.
3)If you worry at all about anything to do with the car, then leave, again it might be a dog in disguise.
4)If anything comes up with the car on the test drive, and the dealer claims they will fix it, then leave, they might not actually do the fix, and might just cover up the fault. Not worth the risk.
5)If a dealer offers an MOT, just treat that as a saving of £50 or whatever on the car and a saving of the hassle of having to get an MOT done any time soon, MOTs done by people who are selling cars may not be a good indicator of the state of the car.
6)If a dealer claims they will do a 'full service' on the car, expect probably a wipe of the windscreen and maybe if you're lucky an oil change, don't expect anything like inspecting brakes, spotting/fixing obviously completely shot wheel bearings etc etc.
There are tons of cars out there - you are much better off buying a car that seems okay from someone who seems trustworthy, to minimise the risk of buying something dodgy. If they seem like dodgy second hand car dealers, then just walk away, totally not worth the risk.
Oh and if you're buying from a dealer, always google for 'dealer name' reviews before even looking at any cars. If there are any bad ones, then don't bother.