To estimate a maximum purchase price for the coin in question, we will 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. Starting Price: The starting price is 1800 euros, which is a good starting point.

3. Auction History:
- The average hammer price in previous auctions is 1506.67 euros.
- In past auctions, hammer prices have varied, but there are examples of coins with similar conditions that have sold for higher prices, such as 2800 euros in a recent auction.

4. Price Trends: The overall trend appears to be upward, especially for coins in good condition. The last auction with a hammer price of 1800 euros and an estimated price of 3000 euros suggests that there is significant interest in this coin.

5. Comparison with Previous Prices: The coin has had hammer prices ranging from 1100 to 2800 euros in similar conditions, indicating that the market is willing to pay more for well-preserved coins.

Taking all these factors into account, an estimated maximum purchase price could be around 2500 euros. This is based on the good condition of the coin, market interest, and previous hammer prices.