that's too general as you didn't gave any info about your team or your targets.
So,
If you have a lot of greens, buy a young fast trainer, give him also AML or AMR position, maybe a sa too and make a rotation with your other ST- winger.
If you don't have many sources, buy a simple ST with not the higher quality.
For example, if your 6* players have 35 quality, buy a 26-29 y.o. player, with 33 quality , so at the end of the season he 'll be 34 q and you can sell him at the end or the beginning of the new season easily.
(a simple 35q player cannot be sold easily).
Don't go for sa cause this attracts many bidders.
If you find someone with good stats (2 goals in 3 games) go for him.