It's always worth it to get higher quality. It usually lasts longer, get the best you can afford comfortably. Bike shops usually have an extremely low margin on bikes, with prices set to a degree by manufacturers. Just about any bike at a bike shop will cost what it's worth, some brands do represent a better price point value, however. Accessories are the only place products vary in price, and that's often due to the speed they move through the store. In general, no shop will be price gouging on accessories. If you go to a few of them you'll get an idea of what stuff costs. I picked my shops of choice based on the staff more than the products, however. Having a good LBS behind you is invaluable.
Do spend money on wheels, this is one place where weight limits are surprisingly low. I'm at 190 and marginally overweight for a lot of wheels, and it's my experience that wheels I'm on the limits of don't last terribly long on the awful backroads in my area. Get a set of handbuilt high spoke count wheels and you'll probably never need more. Tell shops your concern and see how they suggest you go for wheels, many shops can build an excellent set for you, with spoke type and quantity as well as rim geometry suited to someone who is going to test their limitations.
Back pain is symptomatic of fit, pay for a professional fitting when you get the bike and you shouldn't have problems.