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

1. Starting price: 20 euros.
2. Average hammer price: 45 euros, which indicates that, in general, coins of this reference tend to sell for a price higher than the starting price.
3. Condition: The coin has a condition rating of 90 (EF), which is a good condition and can positively influence the price.
4. Auction history: It has been auctioned once and sold on that occasion, suggesting that there is interest in this coin.

Given that the average hammer price is 45 euros and considering that the condition is high, we could estimate that the maximum purchase price could be in a range above the average hammer price, but not too high to avoid the risk of overpaying.

Taking all these factors into account, an estimated maximum purchase price could be between 50 and 60 euros. This would allow for competitive bidding in the auction without exceeding a reasonable limit based on the price history and the condition of the coin.