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

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 price compared to coins in lower condition.

2. Auction History: The coin has been auctioned 5 times, and all sales have been successful. This suggests that there is consistent interest in this coin.

3. Previous Hammer Prices: The previous hammer prices are:
- 320 (2020-11-19)
- 261 (2020-01-14)
- 320 (2022-04-27)
- 400 (2024-12-11)

The highest hammer price in the past was 400 euros, which sets an upper limit for the price of this coin.

4. Starting Price and Average Hammer Price: The current starting price is 350 euros, and the average hammer price in the past has been 344.2 euros. This indicates that the starting price is aligned with historical performance.

5. Price Trends: Given that the most recent hammer price was 400 euros and considering that the current coin's condition is good, it is reasonable to assume that interest in the coin could lead to a similar or even higher hammer price.

Based on these factors, an estimated maximum purchase price could be around 400-450 euros, considering that the interest in the coin and its condition could justify a higher price than the previous highest hammer price. However, it is important to keep in mind that the market can be variable and that other factors, such as demand at the time of the auction, will also influence the final price.