After the battery of the laptop died and bloated even the case of the Mac, I felt like I have to do something. However, with the proper guides this is no problem at all. So I did
- Change the battary (guide on ifixit.com)
- Upgraded to an SSD disk (guide on ifixit.com)
- Added RAM (guide on ifixit.com)
- If your RAM modules have a higher frequency then your machine can handle and your Mac get stuck in the middle of the boot process, try to upgrade only one module. This will help (gives you a bit more memory, but keeps your machine running), at least until you have proper modules.
- Install the new mac OS Sierra using a bootable USB stick (german guide on macwelt.de)