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

1. Current Condition: The coin has a condition of 75 (MBC), which indicates that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction History: By looking at previous hammer prices for coins with similar conditions (75 and 80), we can see that:
- In past auctions, hammer prices for coins with a condition of 75 have ranged between 25 and 80 euros.
- The average hammer price for coins with a condition of 75 seems to be around 40-60 euros.

3. Starting Price and Average Hammer Price: The current starting price is 25 euros, and the average hammer price in the past has been 43 euros. This suggests that the hammer price could be higher than the starting price.

4. Auction Trends: Given that the coin has been auctioned 18 times in the past and has had a good sales percentage (14 out of 18), this indicates a consistent interest in the coin, which may increase its value.

5. Estimation Based on Previous Prices: Considering that the highest hammer price for a coin with a condition of 75 was 80 euros, and that the average hammer price is 43 euros, we can estimate that a reasonable maximum purchase price could be in the range of 60-80 euros.

Conclusion: Based on all these factors, an estimated maximum purchase price for the coin in question could be approximately 70 euros. This sits at a midpoint between the average hammer price and the highest observed hammer price, taking into account the current condition of the coin.