I remember seeing a teshuva about this once. Could be it was even here. If someone has daled minim and one of the minim is not "mehudar" does it then "take away" from the hidur of the other minim. That because this one is not mehudar it can no longer be called "mehudar daled minim". Thus, if someone has the option to buy one min that is mehudar even though the other ones won't be, is there an advantage in this?
I am not sure I will be able to locate the particular source you refer to, though my perusal of the Yad Moshe index would imply that it isn't in the Igros Moshe. If I find something I'll try to include it.
It would seem, however, that if one is able to obtain a single min (species) that is mehudar one should do so. Practically at my own Shul, and I imagine many others, the only difference between the standard and deluxe arbah minim is whether the Esrog is mehudar.
This seems to be fairly straight forward in the Shulchan Aruch O.C. 666:1. The second understanding brought by the Mechaber there would seem to be the primary source (at least as far as it is codified in the Shulchan Aruch) for purchasing mehudar arbah minim. It explains (if I'm reading it correctly) that one should pay an additional third to purchase an esrog with greater hiddur. Although the Poskim (see M.B. 2 citing the M.A.) point out that this applies to other mitzvah items as well, which would in principle include sha'ar arbah minim) the advice to choose the more mehudar esrog was in no way contingent upon the hiddur of the rest of the arbah minim.