To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Current Condition: The coin has a condition rating of 75 (MBC), indicating that it is in quite good shape. This generally translates to a higher value compared to coins with lower conditions.

2. Auction History:
- In the past, the coin has been auctioned 7 times, of which it has sold 4 times. This suggests that there is moderate interest in the coin.
- The average hammer price in previous auctions is 28.5 euros, which can serve as a good indicator of market value.

3. Previous Hammer Prices:
- In the April 2022 auction, the coin with a condition of 75 was hammered at 45 euros, which is a good indicator that, under similar conditions, the price could be high.
- In other auctions, hammer prices have varied, but the highest recorded price for a similar condition is 45 euros.

4. Starting Price and Average Starting Price:
- The current starting price is 20 euros, and the average starting price in the past has been 16.83 euros. This suggests that the current starting price is reasonable and could attract bidders.

Based on these factors, an estimated maximum purchase price could be in the range of 40 to 50 euros, given that the condition is good and there is a history of hammer prices supporting this estimate. However, it is important to note that the final price will depend on the competition in the auction and the interest of collectors at that time.

Therefore, a reasonable maximum purchase price could be 45 euros.