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

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

2. Current and previous prices: The current starting price is 350 euros. In previous auctions, hammer prices have varied, but there are examples of sales that have reached up to 470 euros for coins in similar condition.

3. Price trends: The average hammer price in previous auctions is 347.67 euros, suggesting that the current starting price is reasonable. However, there are also examples of higher hammer prices, especially for coins in good condition.

4. Demand and supply: Since this coin has been auctioned 17 times and has had a sell-through rate of 94% (16 out of 17), this indicates a consistent interest in this coin, which may influence a higher hammer price.

Taking all these factors into account, an estimated maximum purchase price could be around 450-500 euros. This is based on the highest hammer price observed in previous auctions for coins in similar condition and the general interest in the coin.