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

1. Current Condition: The coin has a condition rating of 90 (EF), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Previous Prices: By looking at previous auction prices for coins with a similar condition (90 or 95), we can see that auction prices have varied. For example:
- In previous auctions with a condition of 90, the hammer prices have ranged from 62 to 270 euros.
- The average hammer price for coins in similar condition seems to be around 100 euros.

3. Starting Price and Average Hammer Price: The current starting price is 25 euros, which is quite low compared to the average hammer price of 100.2 euros. This suggests that there is significant potential for the hammer price to be much higher.

4. Auction Trends: The coin has been auctioned 65 times, with a high sales percentage (56 out of 65). This indicates that there is consistent interest in this coin, which may increase competition and, therefore, the price.

5. Estimation Based on Historical Data: Given that the average hammer price is 100.2 euros and considering that the coin is in good condition, an estimated maximum purchase price could be set in a range above the average price, but not as high as the most extreme hammer prices.

Taking all these factors into account, an estimated maximum purchase price for this coin could be approximately 120 to 150 euros. This reflects a balance between the condition, interest in the coin, and historical hammer prices.