To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Current starting price: 2500 euros.
2. Average starting price in previous auctions: 2860 euros.
3. Average hammer price in previous auctions: 4437.5 euros.
4. Auction history:
- In 2018, the coin was auctioned with a starting price of 1800 euros and was hammered down at 2400 euros (condition 75).
- In 2017, it was auctioned with a starting price of 1500 euros and was hammered down at 1850 euros (no condition information).
- In 2016, the starting price was 5000 euros, but it did not sell.
- In 2017, it was auctioned with a starting price of 3500 euros and was hammered down at 3500 euros (no condition information).

5. Trends:
- The coin has had a varying hammer price, but generally, the hammer prices have been higher than the starting prices.
- The condition of the coin is an important factor, and although there is no specific information about the current condition, the history shows that coins with known conditions have had hammer prices that exceed the starting price.

Given that the average hammer price is significantly higher than the current starting price and considering that the coin has a history of hammer prices that exceed the starting price, an estimated maximum purchase price could be set in a range that considers the average hammer price.

Estimated maximum purchase price: It may be reasonable to consider a maximum purchase price around 4000 euros, as this aligns with the average hammer price and the auction history. However, it is important to take into account the lack of information about the current condition, which could affect the final value.