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

1. Current Condition: The coin has a condition of 85 (MS+/EF-), indicating that it is in relatively good shape. This suggests that its value could be in the upper range of previous auctions.

2. Previous Auction Prices: By observing the hammer prices of coins with a similar condition (85) in previous auctions, we find that:
- In the auction on 2022-10-19, a coin with a condition of 85 sold for 220 euros.
- In the auction on 2020-12-16, a coin with the same condition sold for 240 euros.
- In the auction on 2023-03-16, a coin with a condition of 95 sold for 270 euros, indicating that the value may increase with better condition.

3. Starting Price and Average Hammer Price: The current starting price is 150 euros, and the average hammer price in previous auctions is 152.31 euros. This suggests that the hammer price could be around this value, but given that the condition is good, it is reasonable to expect a higher price.

4. Market Trends: Since the coin has been auctioned 51 times and has a history of successful sales, this indicates a steady demand. Additionally, the fact that there have been no unsold coins in the past suggests that buyers are willing to pay for it.

Based on these factors, an estimated maximum purchase price for this coin could be between 220 and 250 euros, considering that the condition is good and there is a history of hammer prices that supports this estimate. Therefore, a reasonable maximum purchase price could be 250 euros.