You need to know that an electroni machine doesn't mean the machine will knit by itself. It is just the same as your punchcard machine, you have to push that carriage. The electronics are only for the built in patterns.
For example where you might have a punchcard that has a diamond pattern on it, that same pattern could be in the electronic machine under what ever number they gave it (there is a book with all the built in stitches).
So instead of putting in the punchcard to select the needles you would turn the machine on and select the pattern number to use and that built in pattern selects the needles, just like your punchcard machine does.
People hear electronic and automaticlly think the machine knits all by itself. That just isn't so. I have a punchcard and two electronic machines and I have to push that carriage the same on all three machines.
Now the other thing you are talking about, the garter stitch accessory, well I think you are talking about the garter carriage or g-carriage as it is called. These do knit by themselves. They come in several different models. For example I have a garter carriage 93 and an 89. These will work for some machines but not for others. These garter carriages are only made for the brother standard machines (and not all of those). At the end of this I will put a link to a compatibility chart for the g-carriages.
There is another thing that will run your carriage called a motor drive but those are very expensive and very hard to find. I think the last one I saw sell went for around $900 and that was a good buy.
Chart:
Daisy Knits