You will get asuma WB fragments slowly over time from survival trial and chase missions. I wouldn't invest a single coupon to get him, since u get him for free eventually. He isnt used very often as well, mainly 9tails/summon fights, any main blitz teams or coins trial. If you dont need iruka right away (hes good early but at your level you soon unlock kage treasure and with it better ninjas) then wait a little, you can get his frags for free as well from refining 5* ninjas.
If I were you i'd slowly invest into sage naruto, since hes the core of wind main (possible to piece him during 4-5 events with close to none coupon usage) and save coupons for power.
Regarding last sentence about your 5$- you are f2p, then your best friend is time. U can get a lot of stuff for free in this game by just waiting and slowly piecing together ninjas etc. I'd avoid using coupons on ANY "situational" ninja fragments, that wont stay in your team for next few months.

Wind main has this "stagnation period" at your level, where everybody seems to be getting new ninjas and you * to the empress build you created at 30 or so level. Don't mind it, just keep playing and piece your team together (saying this for the future, because once you guys unlock kage there will be a rise of new teams that stomp).
As second rare u can get either orochimaru or sage, its not determined. It's up to you now if you want to get naruto frags and try your luck in kage (since you need bee or gaara for meta wind team or mei for upgraded empress team) or keep going into sage and get other ninjas by fragments (note: sage is the easiest to obtain by frags, hes pretty much every week in some event).
Asuma wb isnt neccessary for wind teams, in fact I hardly ever see him in any wind team 50k power+. He isnt used much because most other meta teams can do some serious damage round1 to u and u lose his buff, which is a huge part of his kit.