Questions about the ability to leak / pleasure outside of session?

My question is do you end up with wet spots in your underwear (the leaking)?

It is important to note the third sentence in the Milestones opening paragraph - "All of these milestones may not apply to everyone." so don't expect them all to apply to you. Unless you are already a heavy leaker you may not notice much of an increase in your actual leakage (please see the user poll How much precum (Cowper's gland secretion) do you produce?). Many men do not experience increased leakage at all, so you may not have to deal with any wet spots.

Or have an orgasm by accident (outside pleasure)?

IMHO, NO you will not have an orgasm "...by accident...", nor are you likely to have an ejaculation 'by accident'. Orgasms are largely a result of mental involvement in the whole arousal process, unless you have the intent to have an orgasm it is not likely to occur.

Also any recommendations on which one to buy would be appreciated.

In regards to choosing a first Aneros model, I recommend that you read the following : The Best Aneros model is...
Good Vibes to You !

I think the ability to ooze precum copiously may be related to genetics. I have oozed precum copiously ever since my puberty, certainly when I am sexually aroused or when I wear my favorite fetish items, i.e. jockstraps and athletic cups!

I’d start with a helix syn,great for starting your journey on.
Some guys leak tons during a session,some leak little to nothing.
I leak when aroused and during foreplay,and a lot during sessions,like rumen said,it’s probably genetics.
Ive never had an orgasm by accident.

Pre-Aneros I only leaked a small amount, and now that I can have multiple prostate orgasms I still only leak a small amount. Maybe that will change in the future as my journey progresses, but I can't see any major changes at this point.

I've not had an accidental orgasm. My spontaneous ones are more... controlled. Sometimes my prostate will start to buzz/tingle and let me know it wants to come, and at that point it's up to me to decide to give in or not. It's always a conscious decision and if I decide it's not the right time or place it will go away. I've never had one that my body forced on me without permission, so I wouldn't worry too much about that. Though sometimes I do wake up in the morning getting tickled by the waves of a gentle orgasm. But my body knows it has full consent to do whatever it wants when I'm in bed 😛