When it comes to koi, I have found it is mostly down to the expense and ensuring you have good filtration, water turnover, and good water exchange for when the koi gets up to 28 inches, if it gets that big.
More fish increases the likeliness of more chasing thus more injuries from these chases. If you want to get a good quality koi, then I would wait, do some research, and look for a 2~4 year old male from a good breeder and expect to pay around $300~$900. A 2~4 year old specifically since it is easier to determine fish's gender. Otherwise, you are taking a "educated guess" shot into the dark on the fish's gender. From what I remember, males do not get as big as the female koi since the males have a tendency to want to eat less food while I have seen female koi act like a hoover vacuum when it comes to feeding time. When it comes to serious decisions on buying expensive fish, then I do some research on koiphen.com and submit a request on their forum for breeder recommendations. Also, if you can, post a picture of the fish on that forum before purchasing it. Also, the female koi get chased more often, which leads to more injuries, than the male koi. There are some decent breeders here in the US of A, although not too many, and too many amateur and/or scheister breeders in the US of A that sell too often on craiglist and ebay. So, when it comes to expensive good quality koi, then it is buyer beware. I have heard good things about Kodama Koi Farm, based out of Hawaii (
http://www.kodamakoifarm.com/lover/start.php).
So, with as many fish as you already have in the pond and it sounds like you want a good quality fish with nice coloring, that is what I would do.
Otherwise, I would look into buying some really quite nice shunbunkins from Rain Garden Goldfish, another Hawaii based operation (
http://www.raingarden.us/goldfish.htm).
I actually think now is the right time of year to get something nice, except that might have been about a month ago or so, but I think they will still ship to you. When it comes to the deliveries, it would be best to pick up the package at the distribution center near you rather than allowing the fish to take a ride in the delivery truck.
I always wanted something quite nice, but then that would go against me being a cheapskate when it comes to fish.