Please..... if you even consider buying that forge, just light your money on fire and use it.... that would likely work better.
I gotta be straight up, you simply are not going to get any type of usable forge for that kind of money ($100), whether it's buy or build. If you're going to be a Bladesmith/Knifemaker, you have to realize/understand that there is going to be monetary output in order to get the tools and supplies needed. Save your money up, and buy GOOD tools to begin with..... Two reasons..... 1. You only have to buy them once, rather then spending money time and time again on the same item(s). 2. If you buy a hunk of junk that is poorly designed and built, in any tool, it's only going to hinder your efforts.
The example I always throw out is grinders..... people want a "beginner" machine, thinking it's cheaper to "learn on", and then when it disappoints, the buy something else that is a tad more money, get disappointed with it, buy another, and so on.... until they finally decide to buy one the "top end" 2 x 72" grinders, and along the way, they've already wasted enough money on junk grinders, to have purchased 2-3 "top end" ones!