Based on the provided data, here is an analysis to estimate a maximum purchase price for the coin in question:

1. Starting Price: 300 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 (300 euros).

Given that the average hammer price is also 300 euros and considering that the condition is good, we could estimate a maximum purchase price slightly above the starting price, especially if there is interest in the coin.

Estimated Maximum Purchase Price: You might consider a maximum purchase price between 350 and 400 euros, depending on the demand at the auction and the interest from other bidders. This would allow you to compete without exceeding a reasonable range based on the auction history and the condition of the coin.