Im biased, but for someone who isn't a super high level, i'd recommend a lightning main. You unlock the ability to switch to a different main at level 60, but for the early levels and against weaker players, the lightning main is much stronger than any other.
Of course, keep in mind that at higher levels, it scales off and it's not as good, but I still like it a lot.
More than anything, however, you need to join a group. Events like Convoys need to be done with someone that is strong if you are not strong yourself. Convoys haven't caught up with China's method yet, so you're going up against the top of the server during this time. Join a good group and you'll be better off.
To be good at PvP is less about paying for it and more about getting the fragments to upgrade those characters. Unless you happen to be rich and can * thousands of dollars on getting Tendo Pain and Hashirama. Search around for lineups and formations in the forum that can help you.
As a general rule of thumb, you want 1 healer, 1 damage dealer, and 1 tank. The last slot is open and can be filled with a combo-er (Kakashi, 3* Sasuke), a debuffer, clones, etc etc. You want to create a team that combos in either a circular manner or between two effects.
Now, for Ingots. Before anything, make sure to do the "Test your Luck" event that gives you coupons for ingots. At the very least, you'll get 20 coupons for 10 ingots. Make sure to do it everyday.
I advise you to spend ingots on magatama first. It gives a fixed increase in strength, which is better than refinement or battle armor, which has a high chance of failure.
Next, get your refinement up to 4 on all characters.
Then, for a P2P, mood. Buy those scrolls and get your mood lvl up to whatever you want it to be.
Last on the list, at least for me, is Battle Armor. Just take what you can get from Strong Approaching and don't be greedy.
As far as events go, worry about those as they come. It might be wise to partition off an amount of ingots to devote to events, but you can't tell the future. Who knows what might come up? Play (and pay!) in whatever way you want to.