I am working (slowly) on a Genestealer Cult codex, and I also play with them using Imperial Guard as a "Counts As" force. Here are my thoughts, using the Imperial Guard codex w/ DH and/or WH allies:
HQ
-Iron-Hand Straken or an Inquisitor Lord with the right equipment can both make a passable Patriarch, though neither is really quite what you would really want. (Remember, the Inquisitor Lord must have a Retinue, which can be kitted out to represent various types of bodyguards and hybrids or even young Genestealers.)
-The Primaris Psyker makes a fantastic Magus.
ELITES
-Psyker Battle Squads can make great later-generation Hybrids w/ Psychic powers.
-Ratlings can represent Brood Brother snipers.
-Guardsman Marbo can represent a partiularly well-trained and skilled Hybrid/Brood Brother.
-Death Cult Assassins (DH/WH Codex) make passable "Purestrain Genestealers" (remember, you have to have an Inquisitor Lord in the army to field them - one of the main reasons for using him as a Patriarch.)
TROOPS
-Infantry Platoons can represent Brood Brothers w/ Military Training, while Conscripts fill in the numbers with untrained Brood Brothers.
-Veteran Squads can represent more highly-trained Brood Brothers, i.e. PDF or Imperial Guard veterans who have fallen under the sway of the Cult.
-Penal Legion make great Hybrids, with the different "Desperadoes" rules representing different generations.
FAST ATTACK
-Hellhounds and Sentinels can easily be used straight up as vehicles "liberated" by the Cult.
-Rough Riders can simply represent cultist cavalry and/or bikers.
HEAVY SUPPORT
-Many of the tanks listed in the IG Heavy Support section would work well as "liberated" vehicles in a Genestealer Cult. Some might be more appropriate than others, but most can work. I like the idea of a Cult fielding a Deathstrike Missile Launcher as a "doomsday weapon".
If you are playing a friendly game, and your opponent is willing, one of the most satisfying things you can do is field a small "allied" army using the Tyranid codex. Two Genestealer Broods and a Tyranid Prime with Rending Claws and Scything Talons as your Patriarch will make for a very fluffy addition to the Imperial Guard forces listed above. (Ymgarl 'stealers can be thrown in as well if you have the points.)