To estimate a maximum purchase price for the coin in question, let's 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 with lower conditions.

2. Previous auction prices:
- In 2019, a coin with a condition of 100 sold for 1600 euros.
- In 2021, a coin with a condition of 95 sold for 2800 euros.
- In 2020, a coin with a condition of 95 sold for 1200 euros.
- In 2015, coins sold for hammer prices of 2500 and 2600 euros, although the condition is not specified.

3. Average hammer price: The average hammer price for this coin is 2133.33 euros, which provides a good reference.

4. Starting price: The starting price is 1800 euros, indicating that the seller expects a minimum of 1800 euros.

5. Price trends: Observing previous hammer prices, it seems there is a price range that varies between 1200 and 2800 euros, depending on the condition and demand at the time of the auction.

Given that the coin has a high condition and considering previous hammer prices, an estimated maximum purchase price could be around 2500-2800 euros, as this reflects both the quality of the coin and price trends in previous auctions.

Therefore, an estimated maximum purchase price could be approximately 2700 euros.