What are you using now?
More then likely a Windows PC, whether notebook or desktop.
Honestly there is nothing special about any Mac computer. It is a computer just like any other. I use both types of systems so I have a semi unbiased opinion ( I am a long time Windows user).
The only thing Macs had going for them was there supposed immunity to viruses and malware. Which has now ended and will get worse in the future.
As far as the quality of the overall computer. From reading this and many other Mac forums they are no more or less reliable then any good Windows PC. Both are subject to hardware and software problems. But they cost a lot more money, for the exact same hardware.
The model your friend is selling wasn't a bad computer when it was new, 3 years ago, but now it is 3 years old. It is on the bottom of the list of Macs that can run Lion and more then likely will not be able to run Mountain Lion, the new OS X version coming out something later this year.
Not sure if that system can have 16GBs of RAM installed, I don't thing it can (8GBs tops). In any event 16GBs is way more expensive then 8GBs. 8GBs about $50, 16GBs about $200+.
With a 3 year old notebook computer anything can go wrong with it at any time and with an Apple Mac since there is no more warranty on it they are expensive to fix.
If you really need a notebook computer then I suggest you add about $600 to your current budget and look at getting a Win PC notebook. At least with a New computer, Apple or other, you will have at least the basic original included warranty with it.
If this was just to fool around with and get use to OS X as an OS then I think it's not a bad price. But for a working computer, used for actual work related tasks, you would be better off buying new.