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

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

2. Auction history:
- The average hammer price for similar coins has been 37.09 euros.
- In previous auctions, hammer prices for coins with similar condition (100) have reached up to 120 euros (in the case of an auction in 2019).

3. Starting price and previous prices:
- The current starting price is 18 euros, which is relatively low compared to previous hammer prices.
- The average starting price has been 24.16 euros, suggesting that the current starting price could be attractive to buyers.

4. Supply and demand: The coin has been auctioned 32 times, of which it has sold 23 times. This indicates good demand, although there is also a significant number of coins that have not sold.

Based on these factors, an estimated maximum purchase price could be between 50 and 70 euros, considering that the average hammer price has been 37.09 euros and that there are examples of previous auctions that have reached up to 120 euros.

Therefore, a reasonable maximum purchase price could be 65 euros.