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 quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Starting Price: The starting price is 1800 euros, which is a good starting point for the estimation.

3. Price History: By looking at previous auction prices for coins with similar conditions (90 or close), we can see that in recent auctions, hammer prices have ranged between 1450 and 1700 euros for coins in similar conditions.

4. Auction Trends: In the most recent auctions, hammer prices have shown an upward trend, especially for coins in good condition. For example, in previous auctions, coins with a condition of 90 have reached hammer prices of up to 1700 euros.

5. Average Hammer Price: The average hammer price for this coin is 1365.75 euros, which can also serve as a reference.

Taking all these factors into account, an estimated maximum purchase price for this coin could be around 2000 euros. This is based on the good condition of the coin, the starting price, and the hammer price history for similar coins.