1) On the VAT issue you would pay VAT at point of entry from the U.S. That VAT then becomes part of your purchase cost, just the same as VAT on any goods that you might buy here. You cannot claim that back unless you are registered and as you said, registering while under the threshold would not be sensible at this point. Incidentally the threshold for sale of goods is €75,000, not €51,000.
2) Re the duty, duty is charged at differing rates depending on the goods. That also becomes part of your purchase cost. There is a database of goods where you can check duty rates, its called .