To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Condition of the coin: The coin to be auctioned has a condition rating of 90 (EBC), which indicates that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Previous auction prices: By looking at the hammer prices of coins with similar conditions (around 90), we can see that the hammer prices have varied, but there are examples of auctions reaching up to 400 euros or more under similar conditions.

3. Starting price and average hammer price: The starting price is 350 euros, and the average hammer price in previous auctions is 319.5 euros. This suggests that the hammer price could be in a similar range or slightly higher.

4. Recent trends: In the most recent auctions, hammer prices for coins of similar condition have shown an increase, which could indicate an upward trend in the market.

Based on these factors, an estimated maximum purchase price for this coin could be in the range of 400 to 450 euros. This is based on the good condition of the coin and the hammer prices of similar coins in the past.