Yes Martial would be pretty special. I worry though about getting rid of both our attacking wingbacks in one summer; they were such a big factor in our success last year, particularly in breaking teams down, and while losing Walker for Aurier I can just about handle (though it's by no means a given he'll hit the ground running) losing Rose as well I feel is a bit much. Personally I'd rather we sold him next summer for £70+ mil, ideally abroad, and in the meantime focused on finding a proper replacement and get our pacey wing-forward from the continent for half the price.
Plus it's not a given Martial would immediately slot into our first team, especially given that it looks like we'll be prioritising a 3-atb. Effectively £70mil is a lot to be paying for what could well be, at least at the beginning, an impact sub.
And finally, what happened to not strengthening our rivals? And 2 in one summer? There are better ways of strengthening our team I feel.
Maybe go for cheaper option like Keita, and still can keep Rose. Sell Sissoko to cover.