To estimate a maximum purchase price for the coin in question, 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 quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Starting Price and Average Hammer Price: The starting price is 300 euros, and the average hammer price in previous auctions is 508.17 euros. This suggests that, in general, coins of this reference tend to sell for above their starting price.

3. Sales History: The coin has been auctioned 29 times, of which 26 have resulted in sales. This indicates good demand and consistent interest in this coin.

4. Previous Prices: Looking at previous hammer prices, there are examples of similar coins that have sold for significantly higher prices, especially those with superior condition (for example, a coin with a condition of 100 sold for 2700 euros).

5. Recent Trends: In the most recent auctions, hammer prices have shown a wide range, but there are examples of sales exceeding 500 euros, suggesting that interest and the value of the coin may be increasing.

Based on these factors, an estimated maximum purchase price for the coin in question could be in the range of 600 to 800 euros. This is based on the good condition, sales history, and average hammer price. However, the final price will depend on the competition in the auction and the interest of other bidders.