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

1. Current starting price: 1800 euros.
2. Current condition: 90 (EBC), indicating good preservation.
3. Auction history:
- It has been auctioned 5 times in the past, with 2 successful sales.
- The average starting price in previous auctions was 1337.09 euros, and the average hammer price was 1672.04 euros.
- The highest recorded hammer price was 3064.07 euros in 2012, when the condition was 85.

4. Price trends:
- The coin has had significant variations in its hammer price, ranging from 280 euros to 3064.07 euros.
- The current condition of the coin (90) is better than that of the 2012 auction (85), suggesting it could reach a similar or higher hammer price.

5. Auction expectations:
- Given that there is an auction scheduled in the coming days, and considering the starting price is 1800 euros, it is reasonable to expect that the hammer price will be above the previous average hammer price (1672.04 euros).

Based on these factors, an estimated maximum purchase price could be in the range of 2500 to 3200 euros, considering the good preservation and price history. This would allow for competitive bidding in the auction without exceeding a reasonable limit based on the historical performance of the coin.

Therefore, an estimated maximum purchase price could be 3000 euros.