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 fairly good shape, but not the best possible condition.

2. Auction History:
- The average hammer price for similar coins in the past has been 29.32 euros.
- The condition of the coins auctioned in the past varied, but there are examples of coins with similar conditions that have sold for prices ranging from 12 to 80 euros, depending on the condition and demand.

3. Starting Price and Average Starting Price:
- The current starting price is 20 euros, and the average starting price in the past has been 23.67 euros.

4. Sales Trends: Of the 27 times this coin has been auctioned, it has sold 23 times, indicating a good sales rate.

Based on this data, an estimated maximum purchase price could be set in a range that considers the average hammer price and the current condition. Given that the condition is 75, an attractive hammer price could be around 35-40 euros, considering it is slightly higher than the average hammer price and that the condition is reasonably good.

Therefore, an estimated maximum purchase price could be 40 euros.