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 20 (BC-), indicating that it is in quite poor condition. This generally reduces its value compared to coins in better condition.

2. Previous Auction Prices: By looking at the hammer prices of similar coins in the past, we can see that prices have varied considerably. For example:
- In 2020, a coin with a condition rating of 80 sold for 150 euros.
- In 2019, a coin with a condition rating of 70 sold for 75 euros.
- In 2023, a coin with a condition rating of 10 sold for 400 euros, indicating that even in poor condition, there is a market.

3. Starting Price and Average Hammer Price: The current starting price is 120 euros, and the average hammer price in the past has been 194.8 euros. This suggests that, although the coin is in poor condition, there is interest in it.

4. Market Trends: Given that the coin has been auctioned 63 times and has had a high sales percentage (59 out of 63), this indicates that there is an active market for this coin.

Based on these factors, an estimated maximum purchase price could be between 80 and 120 euros, considering the low condition. However, given the interest in the coin and previous hammer prices, a reasonable maximum purchase price could be approximately 100 euros. This allows for some competition in the auction while also reflecting the condition of the coin.