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

1. Current Condition: The coin has a condition rating of 90 (EF), indicating that it is in good shape. This is a positive factor that can increase its value.

2. Current Starting Price and Previous Prices: The current starting price is 1800 euros. In previous auctions, we have seen hammer prices that varied significantly, but there are examples of coins with similar conditions that have sold for higher prices. For example:
- A coin with a condition of 90 sold for 3000 euros in 2018.
- Another with a condition of 95 sold for 12600 euros in 2019, although this is an exceptional case.

3. Average Hammer Price: The average hammer price for similar coins is 2737.92 euros, suggesting that the market is willing to pay more than the current starting price.

4. Auction Trends: The coin has been auctioned 14 times, with an 85.7% sell-through rate (12 out of 14). This indicates a consistent interest in the coin.

5. Comparison with Previous Starting and Hammer Prices: Most coins with conditions ranging from 80 to 90 have had hammer prices ranging from 1200 to 3000 euros.

Based on these factors, an estimated maximum purchase price could be around 3000 euros, as this price reflects both the condition of the coin and the market interest in previous auctions. However, it is important to keep in mind that the final price will depend on the competition in the auction and the demand at that specific moment.