Jake Buxton, Ryan Stevenson, Andy Graham, James Constable, Ben Alnwick. These are the only players I signed for my Crawley team that won League 2 but I already had some good players such as Josh Simpson, Willie Gibson, Matt Tubbs, Pablo Mills and Sergio Torres who may be worth a look at.
Towards the end of the season look for players whose contracts are expiring (player search) and you should be able to pick up some very good players on a free.
Also look at Febian Brandy, prolific striker if you can afford his wages.