I love my Avante. I like the Towa gauge when adjusting bobbin case tension, but I wouldn't say that I couldn't live without it. Just handy for me when changing to different weights of bobbin thread. Especially nice if you wind your own bobbins.
A good angled paintbrush for cleaning the bobbin area is nice too.
I like the tray they sell to hold all your notions as well. Mine came with the machine when I bought it used. I don't know if it is part of the package, but also very handy. It can rest on the bars of the machine. Again not necessary.
I would definitely get or make some longer leaders and some centered measuring tapes to attach to your bars to keep everything aligned to margin. A roll of painter tape is handy and makes good markers as well.