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

1. Starting Price: 700 euros. This is the initial price set for the auction.
2. Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape. Coins in this category typically have a higher value than those in lower conditions.
3. Auction History: The coin has been auctioned once in the past and sold on that occasion, suggesting that there is interest in it and that its hammer price was equal to the starting price (700 euros).

Given that the average hammer price is also 700 euros and considering that the condition is good, we could estimate that the maximum purchase price might be slightly higher than the starting price, especially if there is interest in the coin.

Taking these factors into account, an estimated maximum purchase price could be between 700 and 900 euros. However, to be more conservative and based on the auction history, a reasonable maximum purchase price could be 800 euros. This allows for some competition in the auction while also staying within a reasonable range based on the provided data.