John, hobby shops are no different than any other business. They want to sell you top of the line products that have the highest profit margins. The $450 Digitrax was probably a Super Chief Xtra. If you are planning a basement filling empire, it's not a bad choice.
For the types of layouts most of us have. The Digitrax Zephyr DCS50 is more than enough to run any small to medium size layout. You can get one at
http://www.modeltrainstuff.com/product_p/dig-dcs50.htm for $144. I just picked one up off e-bay, still new in the box, for $130. The NCE 524025 is the equivalent starter set and it goes for $152 at
http://www.modeltrainstuff.com/product_p/nce-524025.htm. You can sometimes find new ones on e-bay for around $135. These are the two leading systems in terms of number sold. Some people hate one brand or the other but both have faults and both will easily get you started in DCC.
If you want top of the line Soundtrax Tsunami decoders, they will cost right around $100, including the speaker. A much cheaper but still nice sounding option is the Digitrax Soundbug that goes for less than $40, inluding speaker. It doesn't have all the features of a Tsunami or Loksound high end sound decoder but certainly sound more than acceptable to me. You can even download different sound files to match your locomotive type.
The Athearn steam engine will run fine in DC on a DCC equipped layout. Whoever is giving you information on how DCC works is either lying or just plain dumb. You can always run one DC (analog) locomotive on your layout along with your DCC engines, and can have more than one analog engine in a consist. The DC locomotive won't just go flying off down the track.
The good news is that it's relatively easy to install a sound/motor decoder in any steamer. The whole tender is empty so you can install the decoder and speaker in the tender and connect it to the engine with a mini plug that will hardly be noticeable. Some guys mount the decoder in the engine and just the speaker in the tender, again, using a mini plug to make the connection between the engine and tender. I haven't been inside an Athearn steam engine in a while so I don't know if there's space for a decoder in the boiler area or not but either method I described would work. The same holds true for your Bachmann. I'm sure there's space inside the boiler for a decoder since some models come the factory with a decoder installed.
I didn't think sound was any big deal until I head my first sound equipped locomotive. It's just like crack - I was hooked instanly. I've got five engines converted tp sound now and the next up on the project list is a Bachmann 2-8-0. Deisles sound good but steam is just fantastic. Having grown up at a time steam engines were still running, the hold a special place in my heart.