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

1. Starting Price: 70 euros.
2. Condition: The coin has a condition rating of 75 (MBC), indicating that it is in fairly good shape.
3. Auction History:
- It has been auctioned 5 times in the past, of which it has sold 4 times.
- The average hammer price in previous auctions is 108.75 euros.
- In the most recent auction (2023-12-18), the coin was hammered at 40 euros, but in a previous auction (2020-04-29), it was hammered at 250 euros, indicating variability in prices depending on demand and condition.

4. Average Starting and Hammer Price: The average starting price is 50 euros, but the average hammer price is significantly higher (108.75 euros), suggesting that buyers are willing to pay more than the starting price.

Given that the condition of the coin is good (75) and considering the hammer price history, an estimated maximum purchase price could be positioned in a range between the average hammer price and a bit more, given that the coin has a starting price of 70 euros.

Estimated Maximum Purchase Price: It could be reasonable to estimate a maximum purchase price of around 150-200 euros, taking into account the hammer price history and the condition of the coin. This would allow for competitive bidding in the auction without exceeding the market value observed in previous auctions.