When I repotted mine 2 years ago I just used multi-purpose compost with some well rotted manure and bonemeal mixed in and they are doing well. I forked some Vitax Q4 in around the top earlier this spring and I'm keeping them well-watered as it so dry.

For buxus I would use John Innes no 3, unless it is a very young one, in which case ordinary general purpose potting compost will be fine.

I'd use John Innes No. 3 or No. 2 for a smaller plant.