If you have the money, buy both a schoolmaster and a young horse. Training a young horse to FEI is an invaluable experience, however you do need to start getting experience at CDIs.
For a schoolmaster, right now you just need something to do the FEI Juniors, but next year you can bump up to the Young Rider division, so a good PSG horse is advisable.
For a young horse, for your first one mind is everything. You still need something able enough to do a PSG, but always keep in mind that the horse is always for sale. You can find these horses locally, but given your proximity to mainland Europe and the prices right now, it might be worth it to look there as well.
As far as an internship, yes it is an invaluable experience, but a lot of work. As well, I would suggest continuing with school and getting a post secondary degree to fall back on. Until you are out of YR,it will be tough to do the professional rider thing, so just keep learning and at 22 start looking for your internship position. Until then a good local coach and getting involved with the national team through youth competition will suffice.
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