You can always farm out boe sets. It will be painful but may pay off for you in the end
Like pretty sure every piece for the barbaric cloth set is under lvl 20 for as an example.
I'd recommend googling: "all insert class tmogs" there is a wowhead guide for every class and at the bottom of every guide they mention all the tmog set by lvl (1-20, 20-40,etc.)
However if you're a pure f2p plate wearer you're going to have a bad time because I can't think of a single set that's lvl 20 and below :/. You'll have to stick with the old f2p look,which I find cool anywho haha