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

1. Condition: The coin has a condition rating of 100 (MS), indicating that it is in excellent condition. This generally increases its value compared to coins with lower conditions.

2. Auction History:
- The average hammer price in previous auctions is 33.79 euros.
- Previous hammer prices for coins with similar condition (100) have been 40 euros (2020), 35 euros (2018), and 40 euros (2018).
- The current starting price is 18 euros, which is relatively low compared to previous hammer prices.

3. Price Trends:
- The average starting price in previous auctions is 24.38 euros, suggesting that the current starting price is competitive.
- The coin has been sold in 15 out of 18 auctions, indicating good demand.

4. Previous Price Estimates:
- Estimated prices in previous auctions for coins of similar condition have ranged between 30 and 75 euros.

Based on these factors, an estimated maximum purchase price could be in the range of 40 to 60 euros, considering that the condition is high and that the hammer price history supports this estimate.

Therefore, a reasonable maximum purchase price could be 50 euros.