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

1. Current Condition: The coin has a condition rating of 70 (VF-), indicating that it is in acceptable shape, but not among the best qualities. This can influence the price, as coins in better condition tend to fetch higher prices.

2. Auction History: The coin has been auctioned 25 times in the past, of which it has sold 17 times. This indicates a consistent interest in the coin, which can increase its value.

3. Previous Auction Prices:
- The average starting price is 23.67 euros, and the average hammer price is 29.35 euros.
- In previous auctions, hammer prices have varied, but there are examples of sales reaching up to 65 euros under similar conditions.

4. Recent Trends: In the most recent auctions, the hammer price has been around 45 euros, suggesting that interest in the coin remains steady.

5. Current Starting Price: The starting price is 15 euros, which is relatively low compared to previous hammer prices.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 40 to 50 euros. This is based on the history of hammer prices and the current condition of the coin. However, since interest may vary and the auction may attract different bidders, it is advisable to be prepared to bid a little higher if competition is strong.