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

1. Coin Condition: The coin to be auctioned has a condition of 100 (S/C), indicating that it is in very good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction History: By observing previous hammer prices for coins with a similar condition (100 or close), we can see that the hammer price has varied. For example:
- In the auction on 2020-12-17, a coin with a condition of 100 was sold for 70 euros.
- In the auction on 2018-09-19, another coin with a condition of 100 was sold for 14 euros.
- In the most recent auction on 2023-01-18, it was sold for 45 euros.

3. Starting Price and Average Prices: The current starting price is 18 euros, and the average hammer price in the past has been 16.81 euros. However, given that the condition of this coin is high, it is reasonable to expect that the hammer price will be above average.

4. Auction Trends: Since there are 2 auctions scheduled in the coming days, this may increase competition and, therefore, the hammer price.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 50 to 70 euros, considering that the condition is high and there is a history of hammer prices that supports this estimate. However, it is important to stay alert to the dynamics of the auction and any competition that may arise.