If you're quoted less than £70 per hour, go somewhere else.
If you can walk in off the street and get it done right away, go somewhere else.
Even if your tattoo is simple, and you think that there's no way somebody could mess it up, there's plenty of tattooists out there who can mess it up.
I'd agree with your first comment; if it's cheap it's going to be crap. (Although I've had some lucky exceptions)
I disagree with your second comment. I have over 30 tattoos myself and spend a lot of time hanging around with tattooists. They are not busy all the time (especially in the winter when trade really sucks) and it's perfectly possible to sometimes get a good artist to do the work right away - especially small pieces.
I agree with your third point. There are some tattooists out there that shouldn't be let loose with a box of Crayola, let alone a tattoo machine.