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

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape. This is a positive factor that can increase its value.

2. Previous Auction Prices:
- In previous auctions, the hammer prices have varied, but there is a range that can be observed:
- In 2019, it was sold for 1700 euros with a condition of 80.
- In 2018, it was sold for 3600 euros, although the condition is not specified.
- In 2024, it was sold for 4000 euros with a condition of 85.
- The average hammer price of 2860 euros is also a good indicator.

3. Starting Price and Average Starting Price:
- The current starting price is 1800 euros, which is lower than the average starting price of 2033.33 euros. This may indicate that there is a buying opportunity at a reasonable price.

4. Sales Trends: The coin has been auctioned 6 times, of which it has sold 5 times. This suggests that there is consistent interest in the coin, which may influence its value.

5. Maximum Price Estimate:
- Given that the average hammer price is 2860 euros and considering the current condition of 80, a reasonable maximum purchase price could be in the range of 2500 to 3000 euros. This is based on the fact that the condition is good and that the starting price is attractive.

Conclusion: An estimated maximum purchase price for this coin could be approximately 2500 to 3000 euros. This allows for a margin to compete in the auction, considering that the hammer price may exceed the starting price.