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 not in the best shape. This will negatively affect its value compared to coins in better condition.

2. Auction History:
- It has been auctioned 5 times in the past, of which 3 have sold and 2 have not. This suggests that there is interest in the coin, but it does not always sell.
- The starting prices have varied between 25 and 30 euros, and the most recent hammer price was 30 euros.

3. Previous Prices:
- In 2015, the coin sold for 30 euros, and in 2014, the hammer price was also 30 euros. However, in 2018, although the starting price was 30 euros, no hammer price was recorded, indicating that it did not sell.
- The estimated price has been as high as 50 euros in the past, but this may not be relevant given the current condition of the coin.

4. Average Hammer Price: The average hammer price has been 30 euros, which can serve as a good indicator of the current value.

Given that the coin has a condition rating of 25 and considering the price history and demand, an estimated maximum purchase price could be around 30 euros, which is the average hammer price and the current starting price. However, since the condition is not optimal, it may be wise not to offer more than 25-28 euros to ensure a reasonable purchase.

Therefore, an estimated maximum purchase price could be 30 euros, but it would be advisable to consider a more conservative offer, between 25 and 28 euros.