If you're going into a decent bike shop, you shouldn't have too much trouble with being sold a lousy bike. In my experience with cycle shops, the staff is usually incredibly helpful and incredibly knowledgeable about their product. If the bike is selling for less than about $500, you'll want to ask why, same as if you were buying a car that seemed way too cheap... there are some good deals that you can get, but for the most part, the cheaper the bike, the lower the quality. Bikes are a purchase where you truly get what you pay for. Keep in mind when budgeting for this that while the purchase can get a little expensive, it's an investment that should last you about 15 years.
Unless you're going to be consistently riding over razor-sharp rocks, you shouldn't need solid rubbers. A good set of knobbys should suit you just fine. Although, with fist-sized rocks, investing in a full-suspension will almost certainly be a must. My best advice would be to go into your local bike shop, find a helpful staff there, and ask him if you could wrack his brain for a few minutes. Describe what your average ride is going to be like, let him know what area of town you're in, and ask his opinion of what the best bike in the shop is for you.
As a general rule,
Kona,
Giant, and
Trek all make really good bikes for the lower price end of the higher quality scale. Kona really starts to get up there a bit (2-4 thousand), but the other two will be in the $500 to $2000 range for a decent ride.
Don't be afraid to ask to take the bike out for a test run in their parking lot; most shops will allow it, and it's really the only way to get a good feel for how the bike will ride under you. Ask, also, about warranties and guarantees. I've only come across a few shops that will not service a bike that was bought at their shop at a reduced rate, and most of them offer at least a year of free regular service (3 month intervals to check tires, lube, etc.).
If you can make it through this summer and fall on a sub-par bike, or if you don't need a bike until next summer, Oct.-Dec. is without doubt the best time to get good deals on your bike, although selection may suffer a bit, as they're trying to get rid of their old stock. If you're going out in the next couple weeks to buy, you will have fantastic selection, but very little in the way of sales.
One last quick question... why do you plan on walking down the steep hill??? That is the absolute best part of riding, and you want to deprive yourself of it?