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

1. Starting price: 250 euros.
2. Average hammer price: 305 euros, which is the average price at which it has sold in the past.
3. Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape, which may positively influence its value.
4. Auction history: It has been auctioned once and sold on that occasion, suggesting that there is interest in the coin.

Given that the average hammer price is 305 euros and considering that the condition is good, we could estimate that the maximum purchase price could be in a range above the starting price, but not too far above the average hammer price.

Taking these factors into account, an estimated maximum purchase price could be around 320-350 euros. This would allow for competition in the auction without exceeding the historical value of the coin too much.