The features you may want depend on what you want in a machine.
Are you looking for primarily a machine to quilt on? Or mostly to piece on? Or one that does both well?
For piecing only, you can't beat an old Singer Featherweight. Not a new one, only an old one from prior to 1960 or so. They only do strait stitch, come with a single hole plate, are portable, and are all metal. They sew a perfect stitch and with simple oiling and greasing they will last forever. I have two: 1950 and 1951 and both are wonderful to piece on. You can find them on Ebay for around $400 - $800 or on some of the FW dedicated websites.
Many like Bernina's and Janome's too. They each have great features, including needle up/down and lots of stitches. You really have to test drive them... ergonomics are important too! Most machines come with (or you can buy) a single needle throat plate. It is not mandatory to use one for machine quilting, although if you are having trouble with your stitches or corners of the material being sucked down into the wider hole on a regular plate, it could help.
The knee lift is a wonderful accessory but not a must... but if you get used to using one, you'll swear by it. When you push out with your right knee, against the metal bar, it will lift the presser foot for you. If you are sewing and have your machine set for Needle Down, then when you back off the power pedal to stop the sewing, then push the knee lift, the presser foot goes up without you having to reach back there to lift it manually. It stays up as long as you are pushing the bar. So that makes it very easy to pivot when you are sewing and come to a corner. Esp good for machine applique.
As far as a thread cutter, I personally don't like them. They can leave a little tail that then needs to be hand cut off anyway. Double the work.
Take a look at lots of machines, test drive them, and then make a list of what you like and don't like about each. List the features they have and think about which you love, which are optional and which you do'nt need at all.