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

1. Starting Price: 350 euros. This is the initial price expected to be obtained at the auction.
2. Condition: The coin has a condition rating of 75 (MBC), indicating that it is in quite good shape. This generally increases its value compared to coins in lower condition.
3. Auction History: The coin has been auctioned 3 times in the past and has sold on all occasions. This suggests that there is demand for this coin.
4. Previous Prices:
- In November 2020, it was auctioned with a starting price of 200 euros and sold for 300 euros.
- In February 2020, it was auctioned with a starting price of 170 euros and sold for 240 euros.
- The average hammer price in the past is 313.33 euros.

Given that the condition of the current coin is better than in previous auctions and considering the starting price of 350 euros, we can estimate that the maximum purchase price could be in a range above the previous average hammer price, but it also needs to be reasonable in relation to the starting price.

Taking all these factors into account, an estimated maximum purchase price could be between 350 and 400 euros, considering that the condition is good and there is a history of successful sales. A price of 375 euros could be a reasonable benchmark for a maximum bid.