I think the balance you're talking about comes from having a ball playing, sitting defensive midfielder (i.e. a Hamann type). Preferably one who is tall and can help at set pieces too.
The CBs are left exposed far too many times and we all know they're not great players in their own right.
But it's like you say, the system and high energy style we play means we will always concede goals. But it also means we will always create chances, and had Firmino scored that penalty yesterday, I could imagine we would have run out 4/5-1 victors.
A top GK, CB and DM (positions traditionally that don't cost the earth) and I think we will be challenging for all competitions.
But again, like you say, it relies on the patience of the fans (not just to insist we spend for the sake of spending). Klopp might actually target 4th place again, knowing once he assembles the pieces (Van Dijk, Keita, keep Coutinho, get in a DM next summer), we will go from competing for 4th place to competing for 1st. Instead of wasting money and buying a 2nd/3rd choice and still competing for the same 4th position this year (as that player wouldn't be a dramatic improvement).