I play and collect Imperial Fists, as well as their "Archnemesis" army - Iron Warriors. So, I can give you a couple of pointers.
Fluff wise - the Imperial Fists are a very defensive army. They are known for heavy fortifications that are nearly unbreakable. They are also known for their siege warfare skills, since it complements their main purpose very well. It was Rogal Dorn, the Primarch of the Imperial Fists, who was in charge for the fortifications of the Imperial Palace on Terra during the Horus Heresy. The Imperial Fists were the key defensive legion in the process of Terra's defense, alongside the brave Blood Angels and White Scars. Sigsimund, one of the main Imperial Fists' Captains, was in charge of securing supplies, technology and weaponry from Mars when the Heresy erupted. This explains the arrival of the IFs on Mars in the novel Mechanicum.
Imperial Fists make extensive use of Vindicators, Land Raiders, Terminators and Sternguard Veterans. Personally, I built my army around Pedro Kantor - who is the Chapter Master of Crimson Fists, an IF descendant chapter. I painted good old Pedro in IF colors. I have two Sternguard squads and a close combat Terminator squad. They are supported by two Vindicators, a Land Raider (Prometheus) and a Devastator squad. To represent even more reliance on siege warfare, I gave 2 Lascannons to one of my Sternguard squads. And of course, I mechanized the whole army.
If you want to have a special character in charge, then I can recommend Pedro Kantor or Lysander.
In reality they are a very dedicated Codex Chapter. They can easily rival the Ultramarines for their belief in codex organization.