As I understand it engine cases are matched pairs so a mix and match approach is not advisable, they are normally numbered for this reason.
I have had cases cleaned at Sutton Soda Blasting and leave them bare, there is no paint to fall off then.
For an extra few pounds they can also roll the cases in ceramic balls for a while, it gives a nicer finish after cleaning if you don't paint.
If you are going to paint I would suggest looking at
http://www.hi-speccoatings.co.uk/ they can clean and coat as a one stop shop.