If you are not mitering your borders there is no need to sew them together in advance. If you are mitering, I couldn't imagine doing it any other way!
I completely agree with Katier825's comment to think of your borders as you would mats on a print you are framing.
There are a couple of theories regarding measuring. 1) measure through the center only and use that measurement; 2) measure center as well as both sides and use the average of the 3. I tend to only measure through the center. I use a plastic measuring tape and move it along as needed being careful to mark where I am picking up my next point to begin with the tape.
If a particularly long border and I have the length of fabric, I will frequently tear it vs cutting it. I tear about 1/2 wider than I need and press to get all the waviness out then trim down to the width I need. And then I, too, pin beginning in the center, move to the ends and then, depending on how much 'ease' (extra fabric) I might have to worry about, will determine how frequently I pin within those mid-points. Then just sew! If there is a reasonable amount of 'ease' you are dealing with, put the baggy fabric on the bottom next to your feed dogs. This will help to get the extra fabric evenly distributed over the length of your border vs having 'pleats' along the way.
General rule of thumb is to add your side borders before adding top/bottom borders. This is simply because it's a more efficient use of fabric. There is nothing etched in stone.
If using multiple borders, personally, I tend to use a thinner (1.5 - 3") solid color (or a fabric that 'reads' solid) and then a wider outer border. I think the the plain inner border makes a nice visual break between the quilt top and the outer border. These are my personal preferences and certainly not any 'rules'.
We are all supposed to be doing this for fun. Relax and simply enjoy the process. I, too, prefer cotton batting to poly. That is my preference. As to poly vs cotton thread...relax! I use poly thread when piecing. I use whatever thread I have on hand that is close enough in to color to what I need. My machine is not picky. I happen to use cotton thread when quilting but that is because I am a hand quilter and that is what is available for 'hand quilting' thread.
And why would you possibly need to upgrade your year old machine? Is it not performing well? Have you already outgrown it? If you are comfortable with it and it performs well, where is the problem?
If you are having questions about fabric choices for your borders, post photos here. We'll all help!