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

1. Current Condition: The coin has a condition rating of 90 (EF), indicating that it is in very good shape. This generally increases its value compared to coins with lower conditions.

2. Auction History:
- The average hammer price in previous auctions is 24.25 euros.
- The most recent auction with a similar condition (80) had a hammer price of 55 euros.
- Overall, hammer prices have varied, but there are examples of auctions reaching up to 50 euros for condition ratings of 80.

3. Starting Price and Estimates:
- The current starting price is 25 euros, which is a good starting point.
- The estimated hammer price in previous auctions has reached up to 55 euros.

4. Sales Trends: The coin has sold in 16 out of 19 auctions, indicating good demand.

Based on these factors, an estimated maximum purchase price could be between 50 and 60 euros, given that the condition is high and there are precedents for hammer prices in that range. However, to be conservative and considering the starting price, a reasonable maximum purchase price could be 55 euros.