Ok here is advise from me, a professional, dont worry i wont charge you.
First of all, let me just answer your question. For you who will be doing town/city and motorway driving, 0-60 time is probably the best indicator of performance. Age of a car is no way correlated to performance, new cars come in all sorts of varieties, small, big, fast, slow etc. I would not recomend a ford fiesta 1.2 if being tailgated is an issue, I havent looked up the specs but it will be very underpowered, especially with all the car manufacturers doing all this eco friendly stuff like cylinder deactivation or even removal in some cases.
If you want to 'keep up' with the general traffic as I think you want to, I would recommend a 1.7 diesel of the vauxhall/ford. Diesels have generally good 0-60 times due to the power available at lower revs. They also offer excellent fuel economy compared to petrols, so it would be my personal recomendation to you. With that sort of 1.7 diesel you should be getting around 10-12 seconds from 0-60, thats enough to keep you from slowing others down.
If your needs were to extend to track racing or drag racing then 0-60 data is insufficient for your needs. There are too many factors to give an easy answer, the right answer is probably there is no right answer. You can compare two cars, the first will have a greater 0-60 time, and the second will have a greater 60-100 time, so which is faster? Some people are saying bhp/torque are indicators, nonsense. The torque/bhp varies across different revs, it is not fixed and importantly one must consider weight as it is a very very important factor.
You can look at a bmw 335d, it is a diesel and has a 0-60 time of about 5.25ish seconds, debatable, some say more some say less. This is equal to or faster than some proper rear wheel mid/front engine super cars. Does that mean it is 'faster' than a super car? Well from 0-60 on a straight maybe yes, but on a racing track when you take the weight (diesels are very heavy engines), braking, cornering, grip, tires etc the sports car comes on top. Bottom line bhp/torque and 0-60 are not the final word as people here are proposing.
Back to your question and to summarise, modern cars can be very sluggish, or very very super fast, depends on what you go for. Yes 0-60 is the right thing to look at for your needs. And you are looking for 0-60 time absolutely no greater than 12 seconds to not get tailgated.