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 fairly good shape, but it is not the best possible condition.

2. Historical Prices: By looking at previous auction prices for coins with similar conditions (75 and 70), we can see that auction prices have varied but have reached significant figures. For example:
- In 2018, a coin with a condition of 75 was auctioned for 200 euros.
- In 2022, another coin with a condition of 75 was auctioned for 270 euros.

3. Average Auction Price: The average auction price for this coin is 159 euros, which gives us an idea of what buyers are willing to pay on average.

4. Starting Price: The starting price is 90 euros, which may attract more bidders, but it can also indicate that the seller expects a lower price initially.

5. Auction Trends: The coin has been auctioned 18 times, of which 15 have sold. This suggests that there is a consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be between 200 and 270 euros, given that it is the range observed in previous auctions for coins with a similar condition. However, considering the starting price and the interest in the auction, a more conservative maximum price could be approximately 220 euros. This allows for competition in the auction and reflects the historical interest in the coin.