5 - 30 minutes
Replace logic board; replace HDD with SSD; move HDD to optical bay; upgrade RAM.
All in all, everything went well. Considering that this was my first repair ever, it took me much longer (4-6 hours) than indicated (40-60 mins). Also, during the repair I had to watch a couple of video tutorials on YouTube, because sometimes the guides alone weren't sufficient.
To anyone who's going to do this repair, make sure you also get a 5.5 mm Phillips screwdriver. During the repair I came across a step which required this particular screwdriver, but I didn't have one since it wasn't mentioned on the list of required tools of the logic board replacement guide.
Furthermore, there is a Phillips #1 Screwdriver listed as a required tool for this repair, but I didn't use it since I managed to carry out that particular step using a Phillips #00 Screwdriver instead.
Also, at the end of the logic board replacement guide it says: "To reassemble your device, follow these instructions in reverse order". This is slightly misleading, simply because in doing so you would omit a very important step, namely to apply new thermal paste first. Therefore, I would advise to either include or refer to the 'Applying Thermal Paste' guide at the end of the logic board replacement guide.
Phillips #1 Screwdriver
Mac mini Logic Board Removal Tool
Magnetic Project Mat
TR6 Torx Security Screwdriver
MacBook Pro 13" Unibody (Mid 2012) 2.9 GHz Logic Board
Arctic Silver Thermal Paste
Arctic Silver ArctiClean