When choosing sun corals try to pick those as fleshy as possible. The less white skeleton exposed the better. You will have to target feed every single polyp almost every day.
As for anemones do not choose shrunken ones, nor those with inverted mouths, nor those that have trouble anchoring to substrate in the holding tank. Don't choose any with visible damage like tears, etc. Hard for them to recover in captivity. Take great care when removing them from the glass wall. Best is to buy it together with the rock it is attached to, if any.
If you are new into the hobby, I strongly suggest you avoid anemones and sun corals for now. You will save yourself alot of money and headaches.