I bought a Trek 15 speed a few years back that was in the $350 range, topped about $425 with the accessories I added (saddle bags, kick stand, speedometer, tool kit, flat repair/tubes) I test rode a bunch of different makes and models before choosing the Trek. Trek and Cannondale, I believe, were basically the same bike/company, just different branding. I test rode the two side by side and like the Trek just a tad bit more, so that's what I chose.
I am of the belief that a bike should be easy to ride. It should be easy to pedal, no matter what gear you're in, as you grab higher gears you simply go faster for the input. Now, obviously, you're going to work harder as you crank up the gears, but in my opinion, should still be a relatively easy ride. As a comparison, I started with a Huffy from Costco, you could barely turn the crank in first gear and felt like you were fighting through mud just to coast.
Tire size, air pressure, and gear ratios will play a big part in how a bike feels when you're riding it. Of all the bikes I test rode, easily 7 or 8 different brands and models, the Trek was the only comfortable, easy bike to ride. I would encourage you to go test ride as many brands/models as you can and choose the one that feels the best to you.
I would also encourage you to invest in a really good seat! While the stock seat on the Trek was ok, it did get rather hard and uncomfortable after a few miles. A good bike shop can fit you to the proper sized seat based on the sized of your skeletal butt print. You will be looking at about $100 for an upgraded seat, but if you plan to ride this thing more than a few miles at a time, you're going to want something that isn't going to make your butt/crotch ache.
I would also encourage you to accessorize with at least a small bag for basic tools and supplies for flat repairs. Going off pavement, even on bike trails subjects you to all kinds of flat inducing objects. My worst was having to ride through star thistle chaff, which resulted in over a dozen punctures to both tires within seconds! I was able to repair and get back on the road because I had the tools and supplies. Tire spoons, patches, spare tubes, and air pump are the basics, IMO, but you can certainly add as much as you like. Also having the ability to carry plenty of water is a must!
I didn't do a lot of offroad with my bike, but did take it camping and it did pretty well on steep gravel grades, not a fun ride, but it would make the climb. For on road and suburban bike trails, it was more than adequate. the 15 speeds went low enough to easily climb grades, in fact, you could hardly balance because your speed was so slow. The high gears were plenty big to cover decent distances in reasonable time. My average speed was around 14mph and I primarily used the 2nd gear range for all my city riding. My bike had 3 crank sprockets and 5 rear sprockets.