Taking it from what you said - you want to sell the non essentials - then you are the only one that can decide which are essential and which aren't.
It's easy to say treat it like a business, and make the decision from that, but that is hard to do, at least for me!
We're reducing our herd number, slowly. To good homes and/or those that I know I can let almost any of my horses go, but I have a hard time with anything else! I'm delivering one on Sunday to the other end of California - it's hard and I'm chosing not to think about it. Otherwise I'll want to cancel the deal!
It helps if you know exactly how many minis you want to have and that you are comfortable keeping. If the number is 5, 14 or 60, you can work from that.
I would first figure how many horses total, then deduct the number of 'absolutely cannot be sold for whatever reason' ones. This will leave you with a number to work with in deciding where to make cuts in your herd.
I tend to take out ones that I have to many from the same line, example a mare and I have three daughters. Two from that group of four can go, depending on the sire, which ones have the specific usefulness to me, which one I can't sell (usually my older mares!) will decide which go - usually the youngest ones!
I wish I could cull like some people can, Alas, I can on paper or in my head, but not in my heart!
Good luck.