No it doesn't need an external DAC, a lot of people use one because they believe it nets them better sound, I would say sometimes it does and sometimes it doesn't.
As to your second question honestly nothing, although we start getting into better quality. There are those that believe that the more money you spend the better quality you get and the better quality you get the better sound you get. Some would say that the sound quality gained from the cd6007 is far better because of its design and construction and even its internal DAC is better than what can be obtained from The tascan model you mentioned.
In most cases I would agree, I think something like the Marantz CD player is designed to extract as much information and sound quality from a CD as possible, whereas the tascam is designed to be a general purpose machine. Not to say that the tascam won't sound good, I think a lot of component choices and these sorts of things come down to personal preference. Over the years I've heard equipment that The standard enthusiast would dismiss, but actually sounds fantastic.
And that's sort of the Crux of the whole thing. This whole hobby is nothing but preference, some people will go all out and be happy and some people will get the same equipment and be completely unsatisfied. I remember asking myself several years ago why this was the case, and I came to the conclusion that it's because we all hear differently and at the end of the day what you should do is go with the equipment that makes you happy, that makes the sound that makes you happy.
My own personal belief is that better equipment more expensive equipment doesn't necessarily mean better sound. And that is important to find the type of sound that you enjoy and then go with the equipment that makes that sound, whether that be expensive gear or inexpensive gear or some combination of both..
Sorry to be so verbose but just thought I would share my opinion and experience