Hi eash,
Most second hand cars are reliable provided they've been regularly serviced and maintained. There are so many people today - and I hate to say it but most women owners - who do absolutely nothing to a vehicle other than drive it. They never look under the bonnet nor perform even the most basic service checks. Of course, when the car packs up they then blame the car! When people ask the question "What make of car is most reliable?" they invariably mean "What car can I just drive and do nothing to?". Well, the answer is none as all cars need regular servicing and care. A 15 year old car that's always been looked after will be far more reliable than a 5 year old car that's never been serviced.
If you're going to see a car for sale then take someone along who has some idea about them. As a general rule, provided there's no rust and the engine is good then any other problem that appears will be fairly straightforward. MoT tests are very strict today and any car that passes will almost certainly be in reasonable condition. The days of 'old bangers' passing tests are long gone.
In short, you won't go far wrong if you only consider a car that has :-
a) Just passed an MoT
b) No rust and a good engine
c) Has proof of regular service and repair.
Once you've bought a car then you do have to lift the bonnet every week to make basic checks. Provided you keep the car well serviced and maintained it'll probably never let you down, regardless of age.