Guienas having a propensity for eating ticks is a bit of a myth. They do eat a wider variety of insects than regular domestic poultry, plus they tend to cover more ground during the day.
As R2elk said, they need a secure coop to sleep in at night otherwise they'll get eaten by predators at night: owls in particular are a problem if they roost in the trees and raccoons, opussums, and skunks if they're nesting on the ground. Of course, the same goes for ducks.
You'll need to train the guineas to stay on your property: if you just turn them loose they'll likely wander off. They need some reason to stick around which means coop-training them (this takes about six weeks) and occasional treats or simply having a feeder and waterer option.
Even when they do think of property as "home" they'll tend to wander during the day anyway. My 16 acres wasn't enough for mine and they frequently liked to try invading the neighboring farm field, harrass one neighbor's rottweiler, or invade the public road at the end of my driveway. They'd always return from their little adventures after a few hours.
Mine are the bigger "jumbo" French Pearl variety, so improving my fence kept them hemmed in (a 5' tall 2"x4" wire grid fence that they can't roost on top of). But if you get the smaller varieties they're pretty decent fliers and guinea-wrangingling will become part of your routine unless the property is VERY large. The good news is they're quite trainable and readily grasp concepts like pointing and basic hand gestures.
While they don't tend to get human-aggressive like roosters sometimes do, they are semi-feral and very territorial. They'll either run away from strangers or act like little guard dogs and "bark" at intruders of all kinds. People like to say guineas often freak out at nothing all the time, but in reality they're often reacting to things humans can't see or hear.
The one time guineas will get aggressive with humans is when the hens are brooding over a nest and when they have young keets. Usually this behavior is mid to late summer so you want to keep an eye on any kids that want to get "handsy" with the guineas then.
