Not sure how much you have, so I'll just name players that are great but can be cheap and expensive. I assume you won't have that much and you'll not want to get the club in debt so I'll obviously avoid the players like Ronaldo I've put a star next the players I would buy if it were me. Zarate took a season to get going for me, but when he got going he was unstoppable. Rene Mihelic is great mostly because he's so cheap and turns out great (although still highly usable early on in the game), turned into a Wonderkid after 2 season. Bought him for 95k, now worth 7 million after 2.5 seasons. Ever Banega is godlike, definite addition you should be looking at to strength a DMC or MC position.
Midfielder
* Ever Banega - DMC/MC - 4m
Bruno Formigoni - DMC/MC - 300k
Pablo Piatti - MC/AMC - Anything from 100k -> 4 million
Omar Esparza - DL/DMC/MC/ML/AML/AMC - 2.2 million
Left Winger
Sergio Mota - AMLC - Unknown price
* Rene Mihelic - AMLC - 95k I got him for, turned into a Wonderkid
Striker
* Mauro Zarate - FC - 2 - 5 million
Kevin Doyle - FC - 12 million (godlike in this game)
Bogdan Stancu - FC - 400k - 2 million
Genero Zeefuik - FC - 200k (He won't be great, but get for future, he becomes amazing)
* Andrey Arshavin - AMC/FC - 8m (he's the best player in my 2010 Sunderland game)
Midfielder
* Ever Banega - DMC/MC - 4m
Bruno Formigoni - DMC/MC - 300k
Pablo Piatti - MC/AMC - Anything from 100k -> 4 million
Omar Esparza - DL/DMC/MC/ML/AML/AMC - 2.2 million
Left Winger
Sergio Mota - AMLC - Unknown price
* Rene Mihelic - AMLC - 95k I got him for, turned into a Wonderkid
Striker
* Mauro Zarate - FC - 2 - 5 million
Kevin Doyle - FC - 12 million (godlike in this game)
Bogdan Stancu - FC - 400k - 2 million
Genero Zeefuik - FC - 200k (He won't be great, but get for future, he becomes amazing)
* Andrey Arshavin - AMC/FC - 8m (he's the best player in my 2010 Sunderland game)