You should be hesitant, statting your wings would not be a good thing to mess up. One thing is of course to make sure the mana stones you do buy are all the same tier. Another would be that since you probably plan to wear those wings for a long time, get teh best available stats you can get, even if it means delaying your completion. Note that you do not have to stat all 6 at once, you can do it in two groups of 3.
Your question about Stamina versus HP is more convoluted than you might think. One thing is that the stats that are optimum for you depend on your class. Stamina is really good for a knight because he gets bonuses for stamina, however, other classes may benefit more from straight HP. however, again, going back to my first statement, depending on your class you may not want either. A mage for instance is looking for intelligence and Magic attack, a rogue dexterity and physical attack, and so on. There are many threads on optimal statting schemes if you go look in the class specific forums.
If you need to know what any particular stat does for your class, I believe there is a write up in the RoM wiki on that subject.