I started on a 20 button German concertina, then progressed to a 30 button anglo, then a better one, then a 40 button. What you need depends on what you want to do. Obviously more buttons give you more scope, but also cost a lot more money.

I use the extra buttons mainly to get notes in the opposite bellows direction from where they are in the basic 20. That allows you to change bellows direction when it suits the phrasing and/or your choice of chord or harmony, rather than having to change to get the next note.

Try to borrow at least a 30 button one, more if possible, to play with for a few hours. Then you'll probably find more things that you can do, and want to do; and if so you know what you need.

If you want to play like Bellamy, that's up to you, but I've not met anyone else who does. The only person who comes anywhere near that style is Robin Madge.