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 has a condition rating of 85 (ebc-), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Current Listing Price and Previous Prices: The current listing price is 90 euros. The average hammer price in previous auctions is 97.71 euros, suggesting that similar coins have reached prices above the listing price.

3. Auction History: In the auction history, we can observe hammer prices for coins with similar conditions. For example, in previous auctions, coins with a condition of 85 have reached hammer prices of up to 220 euros (as seen in the auction on 2023-03-16).

4. Price Trends: The overall trend of hammer prices for coins of similar condition appears to be increasing, which could indicate growing interest in this type of coin.

Based on these factors, an estimated maximum purchase price could be in the range of 120 to 150 euros. This is based on the average hammer price and the potential for the coin to reach a higher price due to its good condition and market trends.

Therefore, a reasonable maximum purchase price could be 150 euros.