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

1. Starting price: 300 euros.
2. Condition: The coin has a condition rating of 90 (EF), indicating that it is in very good shape.
3. Auction history: The coin has been auctioned 3 times and has sold on all occasions, suggesting a steady demand.
4. Average hammer price: 466.67 euros, which is significantly higher than the current starting price.
5. Previous prices: In past auctions, hammer prices have varied, but the last hammer price was 320 euros, which is a good indicator of its market value.

Given that the condition is high and the average hammer price is considerably above the starting price, it can be estimated that the maximum purchase price could be in a higher range than the starting price.

Taking all these factors into account, an estimated maximum purchase price could be between 400 and 500 euros, considering that the average hammer price has been 466.67 euros and that the condition of the coin is quite good.

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