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

1. Current Condition: The coin has a condition rating of 80 (MBC+). This is an important factor, as coins in better condition tend to reach higher prices.

2. Previous Prices: By looking at previous auction prices for coins with a similar condition (80-85), we can see that auction prices have varied, but there are examples of auctions reaching up to 650 euros for a condition of 85.

3. Starting Price and Average Hammer Price: The current starting price is 350 euros, and the average hammer price in the past has been 295.63 euros. This suggests that the starting price is reasonable and could attract bidders.

4. Auction Trends: The coin has been auctioned 28 times and has had a 100% sell-through rate, indicating consistent interest in this coin.

5. Comparison with Similar Condition Prices: In previous auctions, coins with a condition of 80 have reached hammer prices ranging from 190 to 320 euros, while those with a condition of 85 have reached up to 650 euros.

Given all this, an estimated maximum purchase price for the coin with a condition of 80 could be in the range of 350 to 400 euros, considering that the starting price is 350 euros and there is potential for the hammer price to exceed the historical average, especially if there is interest in the auction.

Therefore, an estimated maximum purchase price could be 400 euros.