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

1. Condition: The coin to be auctioned has a condition of 100 (S/C), indicating that it is in excellent shape. This generally increases its value compared to coins with lower conditions.

2. Auction history:
- The average hammer price in previous auctions is 23.79 euros.
- In past auctions, coins with a similar condition (95) have sold for prices ranging from 20 to 120 euros, with 120 euros being the highest recorded price.

3. Starting price and estimates:
- The starting price is 20 euros, and the estimated price is 25 euros, suggesting that the seller expects moderate interest in the coin.

4. Price trends:
- In recent auctions, hammer prices have ranged from 20 to 45 euros for coins of similar condition.

Based on this data, an estimated maximum purchase price could be set in a range between 30 and 45 euros, considering that the coin's condition is high and there is a price history that supports this estimate.

Therefore, a reasonable maximum purchase price could be 45 euros.