I have several knitting machines, one Singer(bulky weight) and 2 Brothers (one punch card, one electronic). Knitting machines are designed to use a certain weight yarn. There are bulkys that use the bulky type yarn, there are standard machines that use the sport, fingering, dk type yarns and then there are the fine machines that use fine yarn. A fine machine will not knit with bulky yarn very easily, you have to skip every other needle and then it might not work.
Most knitting machines don't like Red Heart Worsted weight yarn. It is too stiff or something, it just won't allow the machine carriage to glide across the bed.
You are also limited to how many stitches wide your piece can be. For instance the standard machine has 200 needles, so your stitch patterns has to fit accordingly. If you wanted to make an afghan on this machine you would have to knit panels and sew them together.
Then there is how to do the ribbing on sweaters or hats. You either need a seperate machine called a ribber that attaches to the main bed or you need a garter carriage or garter bar. With a garter bar you have to manually turn the work over, with a garter carriage you program your rib, 2 x2 or what ever you want then turn on the machine. It will knit the rib (and several different stitch patterns) for your with out you having to move the carriage. The ribber I don't know too much about. I have the garter carriage so I have no need for the ribber bed.
I use my machines to mostly knit stockinette stitch. I don't like setting them up and programing them for the different stitches. But doing a top down sweater is super simple. I can get one done in stocking stitch in about 4 hours. I clamp my machine to either a craft table or the dining room table. This allows me to sit down to work the carriage.
One draw back on these machines is that they are extremely noisy. You can't knit on one and watch tv at the same time unless you turn the tv up very, very loud.
When looking at machines there are machines that you have to hand manipulate the fancy stitches, there are machines that use punch cards and there are electric macines that have stitches programed into them.
The Brother machines are no longer manufactured as are several other brands. Right now I can't remember which ones, but I know that most of them are not made any more. It is really hard to learn to work the machine by yourself. Most of the machines are made in Japan. The manual that comes with most machines are hard to figure out sometimes because they have been traslated from Japanese to English and sometimes things get lost in the translation.
If you can find a "teacher" to help you out that would be great. Also another thing to consider if your machine breaks down it may be hard to find a repair man. I live in California and finding someone who can work on my machine is difficult. However, my Sister who lives in Dallas has no problem because the machine knitting craze is still going on down there. They even have a guild.
There are lots of things to consider when purchasing a machine. Such as figuring out what gauge machine you need according to what weight yarn you will mostly be using. What you will actually be knitting may play a part into your purchase as well. Then there is the pattern capablity. How intricate do you want your stitches.
I am no authority on knitting machines, these thoughts are just what I have learned by working with my knitting machines.