If you are going you should definitely do the 3 passes. They give you some incredible views of Everest and the surrounding mountains. The passes weren't too hard but were by no means easy. The only training I had really was surfing daily. As long as you take it slow and go at your own pace you'll be fine. You don't have to get a guide if you don't want one. Teahouses are free to stay at, as long as you buy your meals from them. I was there in September. May should be a bit cooler, just prepare for that. I wish you the best of luck! Make sure you sit in the front seat for the flight to/from Lukla. The craziest flight I've ever been on.
Everyone has a different reaction to altitude. You can decide on whether to do the passes (all are individually optional) when you get there.
Cho La is over a glacier, so you want to hook up with someone who knows how to avoid crevasses.
This is so beautiful! I would love to visit sometime.