Yes, HG is important, but we don't need HG player in every position. Costa is leaving, the next striker we are going to sign will fill his place. So, no big problem I think.
About Lukaku, yes he score 20+ goals last season, but I have read it before that if you take all his goals, Everton will still stay at 7th position. Almost all of his goals are not giving significant points for the team. So, if we sign Lukaku, you are saying that you will be satisfied if we win every game 1-0 with him scoring, kinda contradicting it.