I personally wouldn't buy a 2nd hand S2. Expecting a phone that's already been used for up to 3 yrs to last another 4 yrs is expecting too much. We see more & more devices with faults nowadays (which is normal for any electronic device with age really), you're almost certain to have some kind of hardware issue(s) at some point. Plus you more than likely have no idea of the 'history' of the phone (what's gone wrong with it up until now).
For me, that would be a no go unless I either got it for free or really cheap (like AUD$25, tops). The only really cheap repair you can do yourself is the USB board which is about 10 bucks, anything else starts to become expensive, like the screen or motherboard, which can as much as or more than what you pay for the phone (depending on how much you pay, obviously). You'd be better off getting something newer/2nd hand.
If you can't afford that, you're going to have to decide whether to take the chance or not, and nobody here can tell you with any certainty how long the phone will last before developing faults (despite what they may tell you; anything people here do say is guessing/bull****), I mean, we've seen phones a few weeks old with faults, and some people have had their phones 2/3 yrs without having anything go wrong hardware-wise. It's the luck of the draw, as the saying goes.