From what I understand of the condition after I came across a Reddit post describing it, it makes the sufferer feel utterly worthless and undesirable due to hormonal imbalances, which can happen instantly. One moment you will be fine and the next you will have serious panic and think the worst. I think it's something they need to know as they can understand a bit more when you start lashing out at them and why you are constantly tired/depressed etc. If they didn't know then they would have a reason to just abandon you wouldn't they as they may think you are being grouchy for no reason.
I think you should tell a partner straight away because they might not want to deal with that and it would be "unfair" to entrap them. It also wouldn't be fair on you. In regards to your last point, it would definitely be easier not to be in a relationship with someone who suffers from PCOS, as you would not have to deal with the constant mood swings and directed at you and/or adapt your lifestyle. If your thinking long term, then you have to consider the possibility that you may not be able to have kids and your partner obviously might want them. Secondly, from what I have read, the partner will have to be a constant "crutch" for the sufferer, and this means taking the lead in most matters of the relationship, and so the partner might want to factor in that you cannot physically work a full time job (tiredness etc). The mood swings can also be dangerous to any kids in the relationship as you may end up picking up a weapon and using it on them.
However, this is by no means from any sort of expert, I merely came across this info on Reddit Lol. I don't mean to cause any offence at all and I hope people don't take issue with me. I am sure a person who is worth being with will stick by you, but it's right that they have all the information to make that decision