bro it's kinda complicated to say which is the best event to spend coupons. Events are losing value for money. Compared to other regions we are way behind in quality of events. Maybe the best one is Lucky Stars + Fuku, lucky gives potion, refining and you can get a ninja or power pack at store, too bad the jackpot is impossible to get. The amount you should get without reloading will vary depending on how often you do the events, it will depend on your rank in the illusion and the releases you do. The average is 10 to 15k per month that you can collect. There is no good ninja to get with seals, if you are a beginner maybe a gengetsu full bt or zabuza edo bt can help in the beginning of the server. I suggest gathering 500 sealings to spend on compensation + Carnival recruitament that gives a ninja (the last ones were horrible so don't expect too much). As for the meta:

They trample everything and everyone: Shisui Susano bt, Ashura bt, Indra bt(coming soon), Guy Gate of Death.

It has its space: Madara Edo, Gaara Swinsuit, Pain Six parth, Itachi edo bt, Jiraiya genma, Nagato winter, Orochi GNW, Zabuza Ronin, Hashirama New Year BT.

Of these being F2P it is only possible Madara edo, Jiraiya, Hashirama ny, itachi edo and Orochi gnw.