Try to get Julian Brandt from Bayer Leverkusen, either permanently or on loan. I had him on loan at Everton in the Championship during season 4 and he was excellent. He's a right winger.
Set your first target on becoming the 3rd best team in Germany, ahead of Schalke, Leverkusen, Stuttgart and the rest. That should take a couple of seasons but you'll be improving your squad with each transfer window and in the meantime, build up your youth academy and get your backroom sorted out so that every area of training is being covered by a specialist and the workloads are manageable. Sign young players with excellent potential to go with the best of your youth academy products and supplement that with the occasional big money mega signing. It will take a while, but you'll be able to overcome Dortmund and even Bayern, eventually.