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

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. Current Starting Price and Previous Prices: The current starting price is 350 euros. In previous auctions, hammer prices for coins with similar conditions have varied, but there are examples of auctions that have reached up to 440 euros under similar conditions.

3. Price Trends: Looking at previous hammer prices, the average hammer price is 310.78 euros, suggesting that the hammer price may be in a similar range or slightly above the current starting price.

4. Previous Estimates: In past auctions, coins with condition ratings of 90 have had hammer prices ranging from 360 to 440 euros, depending on demand and other market factors.

5. Proximity of the Auction: Since it will be auctioned in the coming days, demand may be high, which could push the price towards the upper end of the estimates.

Based on these factors, an estimated maximum purchase price for this coin could be between 400 and 450 euros, considering that the condition is good and there is potential interest in the auction. Therefore, a reasonable maximum purchase price could be 450 euros.