In the end it is up to you, if you want to purchase the one at WallMart go for it. You sound like you know enough to work on some of the things on you're bike on your own. I would suggest against the $10 tires atleast though, they won't wear near as well, or resist flats near as well.
If bike shop does not take the time to try to get a bike your size to fit your correctly by swapping the stem... seats etc don't buy their. Most important is you enjoy the bike, if you feel that the walmart bike will fit the needs buy it, and enjoy riding it and by the time you go you will know whether it will be able to make the trip or not.