To estimate a maximum purchase price for the coin that will be auctioned, we will consider several factors:

1. Condition of the coin: The coin in question has a condition rating of 90 (EF), indicating that it is in quite good shape.

2. Previous auction prices: By looking at the hammer prices of coins with similar conditions (around 90) in the past, we can see that the hammer prices have varied. For example, in recent auctions, the hammer prices for coins with a condition of 90 have ranged from 320 to 400 euros.

3. Starting price and average hammer price: The starting price of the auction is 350 euros, and the average hammer price for this coin in the past has been 323.88 euros. This suggests that the hammer price could be in a similar range or slightly higher.

4. Market trends: Given that the coin has a starting price of 350 euros, it is reasonable to assume that bidders will be willing to pay a bit more, especially if there is interest in the coin.

Based on these factors, an estimated maximum purchase price could be between 400 and 450 euros, considering that bidders may be willing to compete for the coin based on its condition and the general interest in the market.

Therefore, a reasonable maximum purchase price could be 450 euros.