Hi Andyman,
Megapak is definitely a good choice due to the additional equipment features among themselves RealTracks.
If you need RealTraks for a certain style of music, then you better buy the necessary set or RealPAK. All about RealTracks or never use and only those of the files take up disk space.
It is certainly appropriate to buy BIAB version 2017 and, if possible, from the official dealer from a list PG Music so you should be sure that the license key will register on the site PG Music for direct download of all the packages and patches.
UK:
http://www.pgmusic.com/distributors.htm#greatbritain"Musical Progressions" This company, which sells on Ebay UK, I not found between the official distributors.