Take your time and look at several cars. I suppose it depends how flexible you are as to which Z 3 or 4, exterior colour, interior colour and options fitted to the car. If you choose a BMW approved used you will get a 12 month BMW warranty with roadside assistance and MOT cover but you will pay a premium price. If you buy it private or from a non BMW dealer your on your own basically. Having said that I bought my Z4 from a VW dealership and they gave me the same warranty and roadside assistance as BMW would but I'm not holding my hopes up about how its going to work and to be honest I would rather not find out?
Let us know the outcome of your search and look forward to hours of pleasure when you finally find the right car whichever Z it is?