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

1. Current Condition: The coin has a condition of 75 (MBC), which indicates that it is in quite good shape. This generally increases its value compared to coins in lower conditions.

2. Current Listing Price and Previous Prices: The current listing price is 450 euros, and the average hammer price in previous auctions is 320 euros. This suggests that the market has valued this coin in a range lower than the current listing price.

3. Auction History: The coin has been auctioned twice before, with a hammer price of 320 euros in the 2020 auction and a listing price of 350 euros in the 2024 auction, although it did not sell in the latter. This indicates that interest may have decreased, especially with a condition of 25 in the most recent auction.

4. Market Trends: Given that the average listing price of similar coins is 341.67 euros and the average hammer price is 320 euros, this suggests that the market is willing to pay around 320-350 euros for coins in similar conditions.

Taking all these factors into account, an estimated maximum purchase price for this coin could be around 350-400 euros, considering that the condition is better than in previous auctions and that the current listing price is 450 euros. However, given the sales history and the average hammer price, it would be wise not to exceed 400 euros to ensure a reasonable purchase.