If 6 years is half your lifetime, what you need most is a knowledgable adult who'll help you. I suppose here that your parents are happy with this horse/donkey idea, that they are fully supportive and that they know that some things will need to be handled by them and not you.
You wrote something that had me really concerned:
Well, think I decided I'm not ready for fillies :( but from the same seller there's a welshxexmoor mix, for cheaper and she's broken but not ridden too much. She's three. But I've heard that they drug horses to make them seem fine when you buy them. I suspect it's true, how to I watch out for that?
This makes me think that you have a horse-seller near you, and that you're pondering to buy whatever he/she has. This is the wrongest thing you can do! It might work for cats or hamsters, to go to a single place and pick the one you like most. Not with horses!
You (and your parents, riding instructor, mentor, vet-cousins...) need to decide what kind of horse is right for you. Not just "exmoor pony" (in fact, you could find a perfect pony of another breed, or a cross), but gender, age, level of training, disposition towards the activity you'll be doing (jumping? Trails? Barrel racing? A pony can be perfect for one of those things and a nightmare for another). Then,
when you know what you are looking for, you actually look for it, and if your neighbor seller doesn't have your perfect pony, you look elsewhere. You don't have
to buy from this guy, if he doesn't have the right pony for you.
You have 6 years of experience... on well behaved school ponies, under the vigilant eye of an instructor. At home, you would be little more than alone. You've probably never even seen a horse with behavior issues, and for sure you don't have the experience to correct them. You have
to look for an older, well experienced, well behaved pony. No fillies or 3-4 year olds. Young horses/ponies learn too quickly bad habits.
A "wrong" choice would be dangerous for your safety, and would make you fear and eventually hate your new pony. That's why you have to try different ponies, at different places, before you choose. Don't pick the first you see. Try at least 3-4, and then if you (or your instructor-knowledgable adult friends) are not sure, try some more. If you decide you like one, come back and ride him again another day, don't buy immediately. Try all the stuff that you'll do at home, so grooming, hooves-picking and cleaning, saddling, bridling, getting on by yourself, and so on. Don't bring with you enthusiastic beginners, just experienced people that can recognize a scam when they see it, and that can decide with a clear head if the horse they are seeing is too green, or too sick, or reacts dangerously when touched in the ears... the list would be too long.
Never believe the "oh, he has a long waiting list, if you don't buy him right now, he'll be gone next time"- even if it happens, the world is full of horses, you'll buy another one. Don't let the sellers bully you into buying immediately, when you see just the positive.
I hope you'll find all the support you need to make a good choice!