Fair enough. I was just speaking from a medium to long term viewpoint considering he will cost somewhere in the vicinity of €60 million, and not just in terms of Van Gaal's tenure (because his position could be very fluid come next season). Lukaku is a good player no doubt, but I'm not convinced by the argument that he could be a top, top striker; plus I've always had a preference for versatile forwards. If you get Lukaku, then he becomes the landmark #9 for the team for the foreseeable future. He is a #9, and that's where he will play. You will either have to play Martial as an SS in a 4-2-3-1/ 4-4-2, or as an inside forward in a 4-3-3, which IMO would be a waste of his talents come 2017, and beyond - because he is the perfect modern #9, we just have to be a little bit patient with him.
Ideally, United would be looking to sign someone like a Paulo Dybala - not necessarily him, but someone with a similar playing style and profile. Someone who can play as a striker, false 9, #10, and an inside forward - much like Martial. We get a couple players of that profile, and the options would be endless with positional interchange, strikers who are familiar with playing out wide enabling them to drift and open up spaces, their collective dribbling ability, and so forth. Then you can choose whoever is best suited to being your primary #9 - most likely Martial. A bit like even Liverpool when Suárez drifted wide for Sturridge and vice versa.
The reason I'd like someone like Aubameyang (not him, just someone with a similar profile) is that Martial and said player could dovetail beautifully - stretching defenses out wide, dribbling with venom, and so forth - instead of having overly defined roles. It's just that I don't want the club to pigeonhole themselves by acquiring loads of players that might not be ideal fits, or render our setup too rigid a couple of years down the road, especially for that kind of money. Get a versatile, skillful forward nucleus, give them a bit of time to gel, and watch them blossom progressively (under the right management of course - one that would allow them to flourish as individuals, and as a collective threat).