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

1. Current Condition: The coin has a condition of 75 (MBC), indicating that it is in fairly good shape, although not the best possible condition.

2. Auction History:
- The coin has been auctioned 24 times, of which it has sold 17 times.
- The average hammer price in previous auctions is 15.59 euros.
- The hammer prices in previous auctions for coins with similar conditions (75) have been 14 euros (2022) and 15 euros (2020).

3. Starting Prices and Estimates:
- The current starting price is 12 euros, which is slightly below the average starting price of 12.4 euros.
- The estimated prices in previous auctions for condition 75 have been 20 euros (2022) and 25 euros (2020).

4. Trends:
- The overall trend shows that hammer prices have been in a range of 14 to 15 euros for similar conditions in recent years.

Based on this data, an estimated maximum purchase price could be between 15 and 20 euros, considering that the average hammer price is 15.59 euros and that the estimated price in previous auctions has reached up to 20 euros.

Therefore, a reasonable maximum purchase price could be 20 euros.