Hi. Congrats on deciding to bike around!
So there is a big difference between the used bikes and new bikes you are looking at. The old ones are steel frames and probably have friction shifters (the sekai might have indexed on the rear, depends on the year). The new ones are aluminum. Those are going to have a totally different ride quality. I would really suggest that you test ride a few steel and aluminum bikes and see if you like one over the other. I ride a 1977 steel frame and it is amazing. I have yet to find an aluminum bike that rides as good as steel.
The other thing, I totally agree with FrenchFit. The two used bikes you posted are seriously different sizes and that is probably the most important thing to figure out. Here is a link for a general size calculator if you are not sure what you should be measuring.
http://www.ebicycles.com/bicycle-tools/frame-sizer
I am partial to steel frames and I think both of the used ones have a much higher value than the new ones you are looking at. Either way, I would see if your local bike shop has something similar to the nashbar and dawes so that you can test them out.
If you do decide to do maintenance on your bike yourself, here are some good online resources:
http://www.sheldonbrown.com/
http://www.parktool.com/blog/repair-help (park tools also has alot of repair help in video format so wander around their website)
Good luck!