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 85 (ebc-), indicating that it is in quite good shape. This is important because coins in better condition tend to fetch higher prices at auctions.

2. Starting Price and Previous Prices: The current starting price is 350 euros. In previous auctions, the average hammer price has been 279.62 euros, suggesting that the current starting price is relatively high compared to the average hammer price.

3. Price Trends: By observing previous hammer prices for coins with similar conditions, we can see that they have reached hammer prices that vary, but some have exceeded 400 euros, especially in conditions of 85 and above.

4. Previous Estimates: In past auctions, price estimates have hovered around 420 euros in cases where the starting price was 350 euros. This suggests that there is potential for the coin to reach a higher hammer price.

5. Demand and Supply: Given that there are 2 auctions scheduled in the coming days and that the coin has been sold 90% of the times it has been auctioned, this indicates good demand.

Based on these factors, an estimated maximum purchase price could be around 400-450 euros. This is based on previous price trends and the current condition of the coin. However, it is important to keep in mind that the final price may vary depending on competition in the auction and the interest of collectors.