You can't lose stars past 20 though, at 18 you're right in the thick of the portrait farming hell as most will lvl/de-lvl through ranks 20-15, I assume. Ppl in the 20-24 range with non-basic cards are either absurdly bad or they may normally play in a different region and are just getting to 20 for the card back. I do that in eu, I built a budget zoo deck and get to 20 at the end of the month, when its even easier to climb, for the card back. I've lost a grand total of 1 game in the 3-4 months I've started doing that and that was to a priest who had a complete deck.
I'd consider shaman pricey too for someone starting out, they have a lot of good commons, but like no good basics outside of fire elemental and hex. Control warrior is pricey, but aggro warrior is inexpensive, just zoo and hunter are generally better for aggro, but if you like/want to lvl warrior you can make a decent aggro deck.