If you already have the heroes, the standard double purchase event is better, imho. I would wait for a good one.
Jasmine 6 or 7* doesn't matter...you'll use her in CoF and vs HoL Kong only.
Serafine 7* is good, but there is no other hero to farm in Epic Arena shop, right now, so you better bring her to 7* there.
Li Twins are good for HoL only. For them, 6 or 7* make huge difference but meh...
Avior + Malrath: do you need a support/magic ST dps? (mid-fight oriented)
I recommend to go for Malrath 6*, if you really want to buy in this event