I'd heard John Danaher observe that all styles of grappling are pretty much the same, and any differences arise from the set of rules they adhere to.
In this vein, Just adding my two cents (please feel free to correct me):
I. Stance: In Judo, standing with a "bent-over" stance is frowned upon. (The stance is similar to what a freestyle wrestler would adopt.) In most Judo tournaments, maintaining such a stance for 10-15 seconds gets you negative points. Thus, "Judo-Guys" are not used to such a stance while sparring. Feel free to adopt it (just go over the rules of your tournament).
II. Attacks to the legs: About 8-10 years ago, all direct attacks to the legs, while standing, have been deemed against the rules of Judo. Thus, most Judo guys dont know how to defend a wrestling shot, single or double. Feel free to practice these beforehand.
III: Leglocks/Kneebars: Additionally, in the last 8 years, any kinds of footlocks/heelhooks/knee-bars have been deemed against the rules of Judo. Thus, even when a fight goes to the ground, the chaps with a Judo Background do not foresee these attacks. Feel free to work on these in advance.
IV. Gi vs No-Gi: As goes without saying, most "pure" Judo Guys will stick to the Gi divisons. Choosing the No-Gi division will aid you in circumventing any possibility of facing them. Furthermore, as they are taught, most judo throws do not translate well into No-Gi. If a Judo Guy can translate his Judo Throws into No-Gi: he's pretty advanced and he's a Gecko-Roman wrestler, in essence.
V. Most Importantly, if this is your first competition, please have fun. Hell, even if you've competed before, go ahead and have fun anyway.
Bottomline, have fun.