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

1. Current starting price: 25 euros.
2. Coin condition: 70 (MBC-), indicating good preservation.
3. Auction history:
- In the September 2020 auction, the coin started at 15 euros and sold for 42 euros.
- In October 2020, it started at 25 euros and sold for 44 euros, with the same condition (70).
- In April 2024, it started at 18 euros and sold for 40 euros, but with a lower condition (25).

4. Average hammer price: 42 euros, indicating that, in general, coins of this reference tend to sell above their starting price.

Given that the coin to be auctioned has a condition of 70, which has proven to be attractive in the past, and considering that the average hammer price is 42 euros, an estimated maximum purchase price could be set in a range between the starting price and the average hammer price.

Taking these factors into account, a reasonable maximum purchase price could be approximately 45-50 euros. This is based on the history of previous auctions and the good condition of the coin.