Good idea to think of a timeline.
I don't see Mbappe at all as realistic, I'd replace him with Werner.
Haaland I like because I think I read part of his deal at Dortmund is having a reasonable release clause, which we have done well with (Laporte).
I'm not sure about Martinelli, I can't see how ANY top 6 prem club would sell there best young forwards to another prem team.
My go to are Martinez and Werner, easy to justify with there age and especially with Sane/David likely leaving together I don't think we even have to wait for Sergio to leave imo. Just play say Martinez more often as you rest Kevin and give Jesus minutes on the wings. Go with 2 up top as well.
Arsenal were selling us their best young players when we were upstarts and they were coming off the invincibles and the golden age of Wenger.
In 2/3 years time they'll just be Everton or Leicester to us. Europa regulars, no league titles in nearly 20 years.