hmm i got my rtx for 25 dollars. there are different ones, mostly the ones that cost more come with more different bits. your link seems to be the rtx-4, i think i have rtx 1, less accessories. online probably has less selection then store.
rotary is nice in that you can engrave, cut many things like windows and grills, slice groves into worn screws etc. its definetly a multi tool. the badis that it is slow if cutting large areas like windows. steel of course is the hardest, plastic easiest, aluminum is in between. to make a circle cut you gotta make a series of straight cuts, like a complex polygon. it also comes with sanding bits and stuff for grinding down rough edges.
the more elegant solution is to get a normal corded drill, and buy a round hole bit. that cuts a circle right away. you buy a 80mm bit for 80mm fan etc. pretty simple. but you can only cut holes
there are tutorials on the web for each, thats how i got started, i had no clue at all before
eitherw ay, both tools are useful to have around for a variety of uses. i didn't get a hole drill bit thing cuz i'm cheap. the rotary worked ok enough for me

takes a 10-15 min per hole is my guess. you get better as you go along. cutting off grills is easy. and you get get a small drill bit for the rotary tool for making the screw holes.
oh yea, you can also search for fan templates on google. just search for "80mm fan template" orwhatever. you can print em out. those are more dremil friendly since the hole is more of a square with rounded corners then a circle. thats how i do it. dremils are good at straight lines. the corners take a few more angled cuts.