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

1. Current Condition: The coin has a condition rating of 75 (MBC), indicating that it is in quite good shape.

2. Auction History:
- The average hammer price for similar coins in the past has been 51.91 euros.
- In previous auctions, coins with a similar condition (around 70-80) have reached hammer prices ranging from 35 to 130 euros, depending on demand and other factors.

3. Starting Price and Estimates:
- The current starting price is 18 euros, which is relatively low compared to previous hammer prices.
- The average starting price of 31.92 euros also suggests that there is potential for an increase in the hammer price.

4. Recent Trends: In the most recent auctions, the hammer price has been in a range of 25 to 70 euros for similar conditions.

Based on this data, an estimated maximum purchase price could be in the range of 60 to 80 euros, considering that the coin is in good condition and that the hammer price history for similar coins suggests it could reach higher prices in a competitive auction.

Therefore, a reasonable maximum purchase price could be 75 euros.