Ravine trails and 5km street rides sound like a XC type of bike would serve you well. 4" suspension is probably enough, but more doesn't hurt as long the bike pedals efficiently. Generally, the more suspension travel, the worse the bike is for pedaling on the flat sections. This is not just due to the suspension, but also because with more suspension travel usually comes slacker geometry, which hurts efficiency, but makes downhills more fun.
I'm not fully up to date on the latest bikes, but 4" suspension often means a hardtail bike. 5" suspension tends to bring full suspension bikes into the picture. Both types will be fine for the type of riding you'll be doing.
If you're going to ride in terrain with a lot roots and ruts, a 29" MTB will roll over them much better than a 26". Note that 29" bikes have shorter suspension travel, but a 29" bike's travel isn't directly comparable to that of a 26"er.
Have a look at different manufacturers sites to get a feel for the different MTB types and what they're designed for. There's a myriad of bikes to choose from and finding the right bike can be a daunting task. Specialized's web site should be a useful starting point, as they cover a large range (
http://www.specialized.com/ca/en/bikes/mountain#view-all). Something like a Rockhopper may be a good type of MTB for your use.
Find a good local bike shop. They'll find you a good bike and make sure it fits you. And you'll probably be needing their services in the future, so it's good to get a feel for the shops, too.
Your budget should put some nice options on your table. You may find bikes for less, but since you've already budgeted that amount don't feel tempted to go cheaper. Bikes are fun and well worth the investment - you won't regret it.
My two cents.
