Generally, I would ask to see pictures (before and after restoration) of their work. You should be looking at the depth of the vulcanite black and how even the polish is. Also note the crispness of stamping on the stem and stummel before and after. A good restorer, will take the time to avoid polishing over those features to preserve them. It takes a lot more time and effort to preserve those features, but it is especially important on valuable pipes, because collectors will not pay a premium if they are partially removed by buffing and polishing.
As for the bowl, if you are not planning on selling them in the future, make sure they don't ream down to wood. A grain alcohol and salt soak will remove the oils and leave the carbon behind, reducing the chance of damaging the wood. The only reason to take a cake completely off is because you have a pernicious ghost you can't get rid of with alcohol and salt, or you are selling it. Collectors want to see wood to know if there is any char damage that would have a potential to cause a burnout in the future (and that kind of damage lowers the price obv).
That is a pretty basic restore, that would make them smokable, and a touch up to the wax with a polish makes them look nice again.
More advanced services would be fixing issues with the stain / finish, fixing defects in the wood / fills, removing a stinger from the stem, replacing a stem, etc.
A good restorer will want to have a convo about what you want to do with them, and what your expectations of the finished pipe is. From there, they can make suggestions pipe to pipe what you can expect the restore to cost.
Hope that helps! Make sure to post pics, we would love to see what your new collection looks like!
