The TKL and brown switches is really common on manufactured boards. I recommend buying a used one like a Filco or CM QFR with brown switches to see if you like it. If you don't you could probably sell it for very close to what you paid, and then you'll really know what you want on your custom. I find the brown switches are too soft for me, although I used them for years and just didn't know what else was out there. Also I thought I liked the TKL, but it turns out I like 60% better and they're a little cheaper to build and parts are more readily available. Also check out the 75% layout (like Race3, KBD75, Noopoo Choc Mini).
But if you're building a custom straightaway, you need: PCB, case, stabilizers, switches, keycaps, & USB cable for a working keyboard. Optional: plate, LEDs. You'll need solder and a soldering iron to install the switches. Two other specialized tools that come in handy: a switch puller, and keycap puller. Maybe a desoldering pump and solder wick just in case. Once you have your parts, watch some videos on assembling custom keyboards. I found this guide helpful:
http://matt3o.com/how-to-build-your-whitefox/. He's talking about a particular PCB, but most of the content is applicable to others builds.
It might be hard to build a TKL custom for under $100. But google around, maybe you'll find some reasonably-priced parts. 1upkeyboards has TKL kits. They're way more than $100 without switches, though, for a metal sandwich-style build.
Good luck!