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 80 (MS+), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Current and previous prices: The current starting price is 125 euros, which is significantly higher than previous starting prices, which ranged from 18 to 40 euros. However, the average hammer price in previous auctions is 66.25 euros, suggesting that the current starting price may be a bit optimistic.

3. Auction history: The coin has been auctioned 4 times and has sold in all of them, indicating consistent demand. This is a good indicator of the coin's value.

4. Comparison with previous prices: Previous hammer prices are 65, 40, and 35 euros, suggesting that the hammer price could be in a range of 40 to 70 euros, depending on demand and condition.

Given all this, an estimated maximum purchase price could be in the range of 70 to 90 euros, considering that the condition is good and there is a history of successful sales. However, given the starting price of 125 euros, it is likely that buyers will be cautious and the actual hammer price may not reach that level.

Therefore, a reasonable maximum purchase price could be 80 euros.