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 25 (BC), indicating that it is in quite poor condition. This will negatively affect its value compared to coins in better condition.

2. Auction History:
- It has been auctioned 7 times in the past, of which it has sold 4 times. This suggests that there is moderate interest in the coin.
- The average hammer price in previous auctions is 50.5 euros, but this includes coins in better condition.
- Previous hammer prices for coins with similar conditions (30) have reached up to 55 euros, but there are also examples of lower prices (such as 17 euros).

3. Starting Price and Estimates:
- The current starting price is 35 euros, which is higher than the average starting price of 23.33 euros in previous auctions. This may indicate an expectation of greater interest or a market adjustment.
- The estimated hammer price for this auction is 50 euros, which aligns with the price history.

Given that the condition of the coin is low (25), and considering the hammer price history and the starting price, a reasonable maximum purchase price could be in the range of 30 to 40 euros. This is because, although the average hammer price is 50.5 euros, the low condition will likely limit interest and the final price.

Therefore, an estimated maximum purchase price could be 40 euros.