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

1. Current Condition: The coin has a condition of 75 (MBC), which indicates that it is in an acceptable state, but not of the highest quality.

2. Auction History:
- The average hammer price for coins of similar condition (75) in the past has been approximately 38 euros (in 2016).
- In other auctions, hammer prices have varied, but hammer prices of 20 to 47.67 euros have been seen for conditions ranging from 75 to 85.

3. Starting Price and Previous Prices:
- The current starting price is 15 euros, which is quite low compared to previous hammer prices.
- The average starting price in the past has been 26.91 euros, suggesting that the current starting price could be a good opportunity.

4. Sales Trends: The coin has been auctioned 18 times, of which 11 have been sold. This indicates a consistent interest in the coin, which may influence the price.

Based on these factors, an estimated maximum purchase price could be between 35 and 45 euros, considering that the highest hammer price in the past for a similar condition was 47.67 euros and that the current starting price is low. This also takes into account the trend of hammer prices in previous auctions.

Therefore, a reasonable maximum purchase price could be 40 euros.