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

1. Starting price: 1500 euros.
2. Condition: The coin has a condition rating of 95 (EF+), indicating that it is in very good condition.
3. Auction history: The coin has been auctioned 5 times and has sold on all occasions, suggesting consistent demand.
4. Previous prices:
- In 2017, the hammer price was 1900 euros with a starting price of 1800 euros.
- In 2015, the hammer price was 1900 euros with a starting price of 1500 euros.
- In 2015, another auction had a hammer price of 1200 euros with a starting price of 1000 euros.
- In 2021, the hammer price was 1800 euros with an estimated price of 2000 euros.

5. Average hammer price: 1820 euros, which is a good indicator of market value.

Given that the condition of the current coin is high (95), and considering the previous hammer prices, an estimated maximum purchase price could be set in a range above the average hammer price, but it should also be reasonable in relation to the starting price.

Taking all these factors into account, an estimated maximum purchase price could be around 2000 euros. This is based on the auction price history and the good condition of the coin.