To estimate a maximum purchase price for the coin in question, let's consider the following factors:

1. Current starting price: 400 euros.
2. Current grade: 80 (MS+), which is a fairly good grade.
3. Auction history: The coin has been auctioned once before, with an estimated price of 250 euros and a hammer price of 180 euros. This indicates that interest in the coin may be moderate.
4. Average hammer price: 180 euros, which is significantly lower than the current starting price.

Since the current grade of the coin is better than that of the previous auction (80 vs. 75), this could justify a higher price. However, the starting price of 400 euros is quite high compared to the previous average hammer price.

Taking all these factors into account, an estimated maximum purchase price could be positioned between the previous average hammer price and a slight increase due to the better grade. It might be reasonable to estimate a maximum purchase price around 250-300 euros, considering that interest may not be high enough to reach the current starting price.

Therefore, an estimated maximum purchase price could be 300 euros.