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

1. Condition: The coin has a condition rating of 25 (BC), which indicates that it is not in the best shape. This generally reduces its value compared to coins in better condition.

2. Auction history:
- The average hammer price in previous auctions for coins with a similar condition (25) has been 45 euros.
- The average starting price has been 32.88 euros, indicating that starting prices are usually lower than hammer prices.

3. Price trends:
- In previous auctions, hammer prices have varied, but the range has been between 20 and 100 euros, depending on condition and other factors.

4. Current starting price: The starting price is 18 euros, which is relatively low and may attract more bidders.

Given that the coin's condition is 25 and considering the hammer price history, an estimated maximum purchase price could be around 45 euros, which is the average hammer price for coins in similar condition. However, since the starting price is low, some bidders may be willing to pay a bit more.

Therefore, a reasonable estimated maximum purchase price could be 45 to 50 euros.