Depending on your budget, your options may vary. If you are needing to cover a 5 mile radius, you will likely need to install more than one siren, because most sirens are only effective for a mile to a mile and a half, and hilly/rocky terrain will have an even greater effect on the siren's performance.
If you are looking for a mechanical siren, Federal Signal
are the ones to look at first. Federal Signal has a simple range of sirens from high power rotating sirens, to more simple omni-directional sirens. Their equipment is the most advanced and worry free on the market, with two-way monitoring to verify the siren's status at all times. Sentry by contrast solely does omni-directional sirens with multiple models with varying power. Sentry's equipment is more elementary than Federal's, but will get the job done, plain and simple. Both companies do offer battery backup units, which may be beneficial for power outages.
Your other option is refurbishment. This market is a little tricky, because it's difficult to find people close by that actually do this and activation equipment may be a shot in the dark.
Electronic sirens will work, but I don't recommend them for this particular use case.