For one thing, he will not go for £100m. As great as his numbers are, the simple fact is he plays in the Austrian league and Salzburg will most likely sell for a sensible amount. See
@roonster09's post above, post no. 762.
As for your final point about needing to be tested for consistency at a better team, we as a club are not in the position to be turning down talent of this level. Firstly because we simply do not have the personnel that will keep him out of the team already, and secondly, we at this moment do not have the pull to attract elite level players. If we let him move to another club as a stepping stone and he does well there, we allow clubs stronger than us to enter the bidding war and our fans will lament as to why we didn't sign him in the first place. At that point, he may become unattainable for us.
The brackets available to us are really as follows:
1. Taking chances on top talents doing big numbers in weaker leagues, who may prove to be a dud but have ridiculous potential if they work - e.g. Haaland in this case;
2. Seasoned veterans looking for a last opportunity who are very short term, and pretty much past their peak - Mandzukic tier players etc. (could possibly include the likes of Muller here);
3. Mid level
proven players who will get into our team, but do not necessarily have enough to elevate beyond being good players, as they possess visible flaws that will not be removed from their game. Think Lukaku's level when we actually signed him.