It depends on what do you like about the tree peony, Do you have color preference? do you have fragrance preference? do you have height preference? What kind of flower form ( single, semi-double, double, crown etc...) do you like? How strong and floriferous do you prefer? What kind of tree peonies do you prefer (suffruticosa, lutea, rockii etc...)?

You should do some literature and check all the photos of the flowers of the tree peonies in your nursery before making a decision.

Good luck!

The range of colors, heights and growing habits for tree peonies are absolutely amazing! You'll have to browse some of the online sellers to get an idea of what you want and the difference among the different types; I'd browse online even if you plan to buy locally to see what all is out there. Most of the online peony sellers like Hollingsworth and Adelman's have some tree peonies, places like Cricket Hill and Tree Peony Garden have even more pictures to look at.
Planting a tree peony is completely different from a normal herbaceous or even Itoh peony--those you don't want to plant nearly as deep. Tree peonies are normally produced for sale by cutting a stem and grafting it on to a normal herbaceous root. Problem is, if you plant at the depth you would plant that herbaceous root it'll start growing instead of being the food source for the new tree peony. All of that is a cautionary explanation as to why you'll want to plant your new tree peony roots down several inches!
If you look back at the posts from earlier this spring you'll see some truly beautiful pictures of tree peony blooms in actual gardens. Be careful though, they are so gorgeous you'll be bitten by the tree peony bug and won't be able to stop at 1!

Thank you

What I want to know is what should I be looking for in the graft ?

See if the tree peony develop its own root on the graft and plant it deep.

Tree peony is often grafted on the herbaceous peony's root. So you need to plant it deep so that the herbaceous peony will not take over the tree peony, and tree peony can develop its own root system. So if the herbaceous peony emerges from the graft (called sucker), you need to remove it and dig the plant out in autumn and plant it deep.

Also see if it is a first-year,second year or third year graft, the first year/second year graft often will not bloom the next spring. Need to be patient for flowers.

Littlebin thank you, so like any other grafted plant growth from under soil not good.

Tree peonies will also have new growth from under the soil so be careful what you pinch off. The tree peony leaves look much different from the herbaceous nurse root stock so it should be easy to identify. Planting your new roots so that a couple of inches of the stem is buried along with the roots greatly decreases the likelihood that you'll get herbaceous peony growth.