You can get a used Filco for $100 ish or less if you really want. I am an example of that as my first keyboard, used Keyboards really depend on who sold them, in my case it was a very nice guy who was very hygienic. Other than the possibility of being a germaphobe there is nothing really bad about buying used.
On the otherhand Ducky is great as the go to bargain board, although QFR is also cheaper, but hey why not go for quality as well for a little bit more.
But on to what you were asking, why go mechanical? Really there is no difference in gaming, I myself have ventured into the top .5% of LoL and SC2 with a Logitech G105 and a Macbook Pro laptop keyboard. But there are some benefits to going mechanical.
First:
Mechanical is somewhat modular, if something goes wrong, with reading you can go back and fix it, if a switch isn't functioning normally, you can fix it.
Very good feedback when pressing down a key, at least for me Browns provide minimal hand strain from long hours and Black Switches prevent double tapping.
Also, Mouse wise, honestly since DOTA2 does not require any mouse macro's and my roommate is a DOta2 Pro using one of those length of index finger wireless mice, I can't say that it really would be beneficial to upgrade mice. Although I believe the majority of players go with the Deathadder which sometimes goes for $30.
Dureable wise, go for a Ducky Pro
. But if you save up go for a Filco, I have a Filco and I loveee it