The only things the seller matters for is the price and service. I have no idea how Amazon manages service for their large ticket / large item products, you might surf over and see. Who sold me my TV did not really matter since Panasonic covers the warranty, including in-house repairs for larger sets such as mine. I backed up the Pani warranty with a SquareTrade warranty, it was well priced, is also in-house service and extends beyond the pani warranty which is the only point, since SquareTrade coverage does not kick in until the makers warranty expires.
As for cables, I am sure there is debate on this but I go fiber any time I can, except in the case of HDMI which does it all. With fiber it either works or it doesn't, there are no, say marginal issues, there can be quality as in how well the ends are made, but if the light gets through, you get all you can get. On the other hand coax can have quality issues that may impact the quality of the sound, but given that it is digital again I would think, if the bits make from one end to the other and we are talking 3' cables so little chance of issues typical with analog and coax then they are probably equal, so I would say for me optical is a preference and I have never had any issue with optical.
I think you are making a wise decision to hold off on calibration for now and only do it you are unhappy with what you get and think calibration will fix it. Also you can get a Calibration Blu-Ray DVD for about 20 bucks, I generally suggest to the "novice" calibration person DVE (Digital Video Essentials) "HD Basics".
Their are a lot of cal discs out there but I like this one because it explains what each setting does, how they interact and give plenty of reference screens. Also remember if you screw it up you can always hit the "reset to factory settings" and anything you messed up will be back to the original setting again.
You can also hire Geek Squad and many other Cal places and they don't care where you bought your set, they just charge you to do it. The final reason I suggest people try to DIY calibration is that over the life of the set it is not a one time thing or other conditions may change like you paint the walls, get new curtains, move the TV etc, which any may be a driver for needing to calibrate it again, if you DIY, then you'll be able to do it any time you like.
The first time is painfully long, but each time you do it it will be faster and you'll get better at it. The only real advantage pro's have is experience and access to test equipment and service menu's (actually you can usually find out how to get to your service menu if you really want to...)
Just remember that while the discs and pros will lecture you on the objective of calibration is to get the picture to look like the producer wanted it (calibrated to a standard), that the color, brightness, etc is just as much of the content as the images, sounds and words, to that I say, bunk!
In the end what you want is a picture and sound that you like, and you have all of the test equipment needed to get your picture and sound to that, your eyes, ears, and brain. All you really need to know is what knobs to turn to get the effect you want 