USED MacBook Pro 15 late 2011 (i7)

Hello guys! I'm a bit confused if I'm taking a right decision and hope you can critique or support it.

So, I want to purchase a MacBook Pro, but I don't need Retina display and I don't like that in 2012 Apple switch to a 'better' option by fixing RAM and disc drive on motherboard so that it is not possible to make any updates.

Now, there is just one option on the market if one wants to purchase a NEW MacBook Pro with possibility to upgrade HDD and RAM: MacBook Pro 13 i5. But this laptop doesn't have 4-core processors and from my experience (I'm using the same model but early 2011) there are 2 annoying problems that I don't like: 1.overheating issue and 2. HDD cable breaks once a year without obvious reasons.

Meanwhile, my friend offered me USED MacBook Pro 15 i7 2.2 (late 2011) model in a very good condition. I haven't run any tests on it to check what could be checked before purchase, but this might be a better option for me (assuming that it is in a very good condition inside).

Well, in both cases I plan to install 16GB RAM and SSD+HDD instead of DVD drive.

So, I'm ready to purchase USED 15" but I haven't had any experience using it so I don't know if this model has any 'special' problems (like overheating and HDD cable issue in 13") and a bit worried if this is a good choice.

I'd appreciate if you could give me some information about problems this model has and your thoughts if my decision is reasonable and logical. Also, I'd be grateful if someone can share his/her experience on what tests should be run before purchasing a USED laptop.

Thank you.