[Arthur]

For high quality with an eye on showing, I would not by tosai, unless I want to get an award at the Baby level (in which case another set of rules apply).

1) Go to a dealer you trust;
2) The best tosai will come from Japan in the Spring, usually April
(Tateshitas are liquidated in the Fall: not worth wintering in greenhouse,
in Spring, the breeder evaluates what tosai he will likely make the most
money off by growing out in whatever mud pond capacity he has,
the rest is sold off---the ones he thought worthy keeping over Winter);
3) Ask when the tosai actually got in, if less than a month wait or pass
(beside the necessary quarantine, tosai require that much to recuperate
from shipping and be over the inevitable stress);
4) Ask to see tosai in a group, preferably from the same breeder,
some dealers, and here trust comes in, will already have grouped
the tosai by potential (and price); never select a tosai by itself,
unless you are an expert (nobody really is with tosai :-);
5) OK, now you have a group of tosai in the price range you considering;
proceed by elimination (if the dealer knows you are going to spend
money, s/he will work with you);
a) if you know/trust the dealer, ask for assistance in the selection;
b) eliminate all tosai with defect(s) (should not be there in selected
high quality tosai): small pecs, unequal top/bottom tail, ...;
c) select for conformation and growth potential: long head, broad
shoulder, strong tail stop, equally space eyes, ...
6) Only now are you looking for YOUR tosai(s)...
As we started looking at Showa, I will proceed with a Showa
tosai selection;
7) Good bloodlines show their colors late: you are going to look for
a black swan, an ugly duckling;
8) Good hi and sumi come late in high quality Showa, so you are
looking for potential rather than what shows right now;
9) First and foremost, you want to see good shiroji: at this age,
almost transluscent (strong white would scream MALE), a slight
creamy coloring is OK as long as it is uniform (modern Showa
have Sanke blood in them); take out the tosai that do not pass;
10) Then, look for soft beni or under the skin hi for pattern; there should
be a spot or two (even small) where you can evaluate beni quality;
these should be on the head or shoulder (the rest can be under the
skin; if you are looking to grow the koi out, orangy color is best at
this time; look for thickness, not color; you are also looking for
an attractive (to you) hi pattern, either showing or underlying;
11) Now that you have only a few tosai left to look at, consider sumi;
this will be different if you are looking at old-style Showa or the
modern style Showa withh Sanke blood in them (old style will show
Kage unfisnished sumi, while the modern-style will show small sumi
withi kiwa); anyway in all cases, you are looking for sumi showing
in the front half, underlying and a couple spots where you can
evaluate the "fisnished" quality; do not look for sumi pattern: Showa
do change a lot in the first three years; it is not important to have
sumi in the back half of the fish as long as a) there is sumi on the pecs,
and/or b) there is a (little) sumi on the tail: sumi will come on the back
end with time (2-3 years).

I hope I did not bore you with this, and I am certainly not the best councel, (it is a long journey to knwo it all---this never happens) but this will give you a good starting base.