i really recomend this game, its like crack. srsly.
for now i wouldnt spend anymore than $1 than the 100 coin starter packs for each series. It would be much more cost effective to buy $40 of te and then trade that to another player for a 6* than it would be to spend that on a box and get all 3/4*'s.
If you do really get into the game and want to go all out with your wallet i recommend that you learn what types of events this game offers, pick one you really like, with rewards you really like and really under stand the mechanics of that event, drop some $ on boxes for items that will give you a huge advantage ranking and go to town.
dont worry that you dont have allies or 5/6/7* units, they will come. just kick the inactive allies you have and be active yourself. toward the end of this event everyone gets a free 4*. if you put in 3-4 hours a day grinding this event you get a free 5*. every now and again (the last 3 out of 6 events) there have been 6* units that are a fairly easy grind, ie 10-12 hours a day without spending any money, just wait them out.
also dont engage in rmt, you will get banned, buy te from the ingame store and trade that with other players.
edit to say that last weeks event provided 3 easily farmable 5*'s and 2 6*'s, one requiring luck the other just time and resources. just keep playing and learn and good things will come... i havent spend any money on this game, it took me 6 months to get my first 6* and i now haz 7 good ones ^^