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 good shape. This is a positive factor that can increase its value.

2. Auction History:
- The average hammer price in previous auctions is 696.65 euros, suggesting that, in general, coins of this reference tend to sell for significantly higher prices than the starting price.
- In the past, there have been recorded auctions of this coin that reached up to 2739.49 euros (in 2011), although this may be an exceptional case.

3. Starting Price and Average Starting Price: The current starting price is 250 euros, which is relatively low compared to the average hammer price. This could indicate a good buying opportunity.

4. Sales Trends: The coin has been sold in 17 out of 19 auctions, indicating good demand.

5. Comparison with Previous Prices: Looking at previous hammer prices, especially those with a similar condition (90), the lowest hammer price was 600 euros in 2011, suggesting that a hammer price in the range of 600 to 900 euros could be reasonable.

Taking all these factors into account, an estimated maximum purchase price for this coin could be between 800 and 1000 euros. This is based on the good condition, the auction price history, and the demand observed in previous auctions.