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

1. Coin Condition: The coin to be auctioned has a condition rating of 90 (EBC), indicating that it is in quite good shape. This is important because coins in better condition tend to fetch higher prices.

2. Starting Price and Previous Prices: The current starting price is 900 euros. In previous auctions, we have seen hammer prices that varied, but there are examples of hammer prices exceeding 1000 euros, especially for similar conditions.

3. Average Hammer Price: The average hammer price for this coin is 1250.38 euros, suggesting that, in general, buyers are willing to pay more than the starting price.

4. Recent Trends: In the most recent auctions, hammer prices have ranged from 700 to 950 euros for coins with conditions of 90 and 80. However, there is also an extreme case where a similar coin sold for 16500 euros, which could be an outlier.

5. Analysis of Previous Prices: Looking at the hammer prices from previous auctions with similar conditions, the highest recorded price was 16500 euros, but this seems to be an exceptional case. Most hammer prices for conditions between 80 and 90 are in the range of 700 to 950 euros.

Based on these factors, an estimated maximum purchase price for the coin in question could be in the range of 1200 to 1400 euros, considering that the average hammer price is 1250.38 euros and that the coin's condition is good. However, since the starting price is 900 euros, it is likely that buyers will be willing to bid more, especially if there is interest in the coin.

Therefore, a reasonable maximum purchase price could be 1300 euros.