To estimate a maximum purchase price for the coin that will be auctioned, we can consider several factors based on the provided data:

1. Current Condition: The coin has a condition rating of 90 (EF), indicating that it is in good shape. This is a positive factor that can increase its value.

2. Starting Price: The starting price is 225 euros. This is the baseline for the auction.

3. Sales History: The coin has been auctioned 55 times, of which it has sold 42 times. This indicates good demand and a history of successful sales.

4. Previous Prices: By looking at previous hammer prices for coins with a similar condition (90), we can see that they have reached hammer prices that vary, but some have reached up to 340 euros (as in the auction on March 16, 2023).

5. Average Hammer Price: The average hammer price for this coin is 188.56 euros, which can serve as a reference for establishing a price range.

6. Recent Trends: In the most recent auctions, hammer prices have shown an increase, suggesting that demand may be growing.

Based on these factors, an estimated maximum purchase price could be set between the starting price and 20-30% above it, considering the good condition and sales history.

Therefore, an estimated maximum purchase price could be:

Estimated maximum purchase price: 300 euros.

This price reflects both the condition value and the price trend in previous auctions.