Well a backbox is your own preference, you can get anybox of your choice to fit and can even do that yourself with the right equipment
However i and most would recomend that yes you go for a 2 1/2 inch stainless system. This seems the most popular size from who and what i know
Your gunna have to make some enquiries and find the nearest place to you who will make you one, and expect to pay between 300-400 notes for a good job