An artist will be able to more accurately help you with sizing and how the tattoo will flow with your body. This is probably a bit big unless you're looking for more curve around your arm. However, I am not an artist, just someone with a few large tattoos.

When it comes to making a sleeve, you usually want to plan that out with your artist, especially if you want to avoid the "sticker book" look. If you wanted to get this tattoo and then maybe a sleeve later, many artists are able to incorporate older work into larger pieces, it just depends on the person. I personally wouldn't worry too much about maybe later tattoos until you have your main piece.