Hi J, and welcome to the A2OC.
Your question is extremely open-ended. Although the answer is on this forum, it's divided across a few hundred threads. The pros and cons of the A2 are covered here by the huge knowledge base built up over years. However, in an attempt to summarise...
Pros: build quality, fuel economy, cheap tax, no rust, tardis interior, Audi refinements, etc... all the things that probably prompted you to consider the A2 in the first place.
Cons: terrible factory suspension, unreliable central convenience electrics, lack of A2 knowledge at garages, plus a few other little things... But, these things are easily remedied if you, like many, believe that that pros seriously outweigh the cons.
So, which one do you buy...? Well, that depends very much on the individual. The forum has helped many a member decide upon the car for them, but you've got to help us by letting the forum know how you plan on using the car and what your priorities are. How much do you care about spec? Are you one for luxury comforts, or maybe electronic gadgetry? Is power of real importance to you, or are you essentially looking for something that'll just do as many miles for each drop of fuel with minimal fuss?
The A2 isn't a common car and the very youngest is now 8 years old. As such, buying a decent A2 TDI often requires patience and a willingness to compromise. However, prices have come down in the last 6 months, so it's a good time to get your hands on one.
Cheers,
Tom