Here is what I have tended to do, after years of collecting. Discover my study interest (Gokaden, Bizen, Shinshinto, Tadayoshi school...Impressionist, Goya, or whatever...)and buy something that is representative and of the best quality I can afford. The smith, school, period, village... representation provides timeless opportunity for study and enjoyment. If I like what I've found in research, I will buy another, or something that compliments or contrasts the work .
In this way, a collection can become a just few pieces that really "talk" to you. As Ray points out, tastes change with appreciation - as in wines, so a collection may morph over time, but if you stay focused on quality you won't be so apt to "turnover" your purchase.
An interesting collection focus could be - students of the Soden Bizen masters - those of Kanemitsu, Nagayoshi, Motoshige. The collection could be as small as 3, but what a story.