Upgrade: Hard Drive > SSD, RAM. What's to look out for?

Hi dear Peoples,

i need your advises.

I have a Macbook Pro 13" (Mid 2012) and my original hard drive (SATA 2,5", doesn't bear the last update. My Macbook doesn't recognize any more, so i installed OSX 10.8.5 on my external hard drive. I'm not sure if it's just the HD which is broken, or if the Hard Drive cable just doesn't work.

Anyways, the repair shop i went to, gave me the advise to upgrade to a SSD, which he could do for 300€. I looked up some SSD on the internet and found some for half the price. I'm thinkin of replacin it by myself, since all you have to do is to plug the old one out and the new one in and install the OS.

So my questions are, would it be rentable, if i, instead of buying a new laptop, upgrade my HD to an SSD? And if yes, what else could i easily exchange? The RAM (2x2GB 1600Mhz DDR3 is inside right now)? Anything else, Graphics? My Macbook has gotten pretty slow over time, so i would be happy, if you could give me tips on doing anything to help my old friend getting a little up2date.

And do i have to look out for things like the possibility that my Macbook could overheat with the new parts?

If somebody could help me out with a little knowledge and information, that would be pretty awesome.

Thanks and have a nice weekend!