There are times when I feel a border (or multiple borders) will enhance the quilt. There are times I want it larger and a border (or two) seems the best way to do it. And there are times when simply doing the binding next feels like the way to go.
I would definitely sew them together if you are going to miter the corners. That is ever so much easier and more accurate.
As far as upgrading your machine, unless there is a particular feature(s) you seriously want, stick with what you have and enjoy using it till it gives up on you! I personally prefer the vintage straight stitch only machines that have proven themselves for many years, are easy to maintain and purr like a kitten when in use. I may at some point get an embroidery machine and if I do will probably get as many bells and whi stiles as I can, but usually I manage to talk myself out of spending that much.
My advice to you is go with what YOU want to do on YOUR quilt and have FUN doing it!