I'm doing the same as you. I suggest to start by buying a keyboard in the form factor that you want.
Tenkeyless is only missing the number pad. 75% boards are missing the clusters on the right of a tenkeyless. 60% boards are missing the Fn keys as well. Many of these boards have the funtionality of a larger sized board within the Fn layer.
I bought a KBT Pure because pf the size and I prefer the Fn layer to the KBT Poker, although the Poker has a plate, which I overlooked, but don't regret.
Add/replace pieces as you can, but make sure you like the switches. I bought Blacks because they were readily available and while I like the heavy weight I wish it was clicky, so I'm searching for greens.
I suggest that while you're waiting for the perfect group buys and classifieds to pop up, you get some soldering experience. It's difficult to destroy something completely as a novice solderer, but it's also difficult to do a nice clean job and not horribly disfigure the PCB.
Good luck.