Just earlier today I upgraded my Mac Mini late 2012. It took me longer than the 30-40 min. Perhaps an hour. The problem was some info on the video (macsales) was a bit off. For example the power supply cable was a B* to pull from the motherboard. In the video he was doing it so easily, barely touching it and was coming out. It just...pissed me off. I am not kidding, seriously pissed me off.
Anyway, I am not new to hardware tinkering, been doing this for decades, but this whole operation reminded me why I no longer find it fun.
For someone who has no knowledge to computer hardware (hands on experience), I am not so sure replacing both drives is the best way to start. It can easily go the wrong way.
If you have patience and the right tools (you will need a couple more things than what is usually mentioned in such tutorials), including a tool to grab delicate parts and a credit card (to help you place the second hard drive in place), you should do fine. Just make sure you have plenty of light, good eyesight, plenty of room and patience.