Honestly I'm seeing a lot of Red flags here, a 6 month old without having any medical care being sold for $200 - which is far more expensive than any unregistered mini foal should be in this market (honestly that's a rip off). I don't think I'd feel comfortable with this trade - but I understand that when you love a horse you'll deal with any situation - but have you shopped around? Why not go check out some local rescues just to see what else is out there?
This guy you're getting him from, from the little I've heard sounds really shady. I would be concerned buying an animal from someone like you've explained. At least with adopting from a rescue or buying from a reputable
breeder you know the animal is healthy - considering you're not rich I wouldn't risk buying an unhealthy foal.
I'm not trying to dissuade you from buying a horse you love- I have paid more money than I should for horses I love.
As for clicker training, I haven't found a horse yet that hasn't done fantastic with it! You absolutely need to be consistent with your reinforcement. They should never, ever get a treat for mugging - personal space and respect is vital. Personally whenever my horse gets a click+treat I feed the treat behind her chin, so she has to back up a step to get it - this reinforces the space boundaries. If she does what I want, but then comes and invades my space after the click she does NOT get the treat - if she gets more pushy than she should I'll walk completely away. The removal of the reward is a great punishment for rude/unwanted behavior, of course only once in a while for an extreme act.
Typically I teach my horses the only way to get a treat is to turn their head away. My horses do NOT mug people ever, but you neeeeedd to be consistent about it! If they get a treat even once while mugging they're going to keep trying! It's incredibly rude and even quite dangerous for a horse, even a mini to be willing to invade your space, be nippy or paw or strike for food. That's a big No-no!
I'm very excited for you to get your pony
You're going to love him/her no matter which you get! What color is the mini you're looking at and what are do you live in?