I was going to add an caveat about that, but it turns out, that for the iMac 5,1 (the model discussed in the article) RAM upgrades are easy [1] (SSD not so much, but it should still be better than some of its aluminum successors [2]).

That said, I definitely agree and advise: Before you order parts to upgrade things, check whether you feel up for the install procedure.

[1] https://de.ifixit.com/Anleitung/iMac+Intel+17-Inch+RAM+Repla...

[2] https://de.ifixit.com/Anleitung/iMac+Intel+17-Inch+Hard+Driv...