Probably not worth paying for traffic for a free download site. Not likely to get much return for your outlay.
You may try other free methods like forum advertising ,social networking .But if you are ready to spend money then its ok to advertise through google adwords
Paying for anime traffic won't be that expensive. Free methods are usually sufficient though (as above: forum advertising, social networking etc), since there's so much anime traffic flying about it's not hard to get hold of. No harm in testing out adwords though. Maybe get hold of a $50 voucher and see how that goes
You'll likely be blocked from Adwords. There's a lot of anime and manga fans out there who are willing to report you.
If you can make an interesting banner, try buysellads - there are lots of good places to advertise there. If that fails, looke at adbrite
If you have an anime site, no. If you have an anime site that illegally shows or gives anime, then they'll block you from Adwords.