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 a fairly deteriorated state. This will negatively affect its value compared to coins in better condition.

2. Historical Prices: By looking at the auction prices of similar coins in the past, we can see that prices have varied considerably. Most coins with a condition rating of 70 have reached auction prices between 120 and 280 euros. However, coins in worse condition (like the one we are evaluating) generally sell for lower prices.

3. Starting Price and Average Auction Price: The current starting price is 170 euros, and the average auction price of similar coins is 184.96 euros. This suggests that the auction price could be in a similar range.

4. Auction Trends: Since this coin has been auctioned 25 times and all have been sold, there is a constant interest in it, which may help maintain its value.

Taking all these factors into account, and given that the condition of the coin is quite low, an estimated maximum purchase price could be around 100-120 euros. This is based on the fact that, although there is interest in the coin, its condition significantly limits its value compared to coins in better condition.