To estimate a maximum purchase price for the coin that is going to be auctioned, we can consider several factors:

1. Condition of the coin: The coin in question has a condition of 80 (MBC+), which indicates that it is in relatively good shape, but not of the best quality (FDC).

2. Previous auction prices: By observing the hammer prices of coins with similar conditions (around 80), we can see that the hammer prices have varied. For example, in previous auctions, coins with a condition of 80 have reached hammer prices ranging from 260 to 360 euros.

3. Average hammer price: The average hammer price for this coin is 320.47 euros, which can serve as a good indicator of the expected value.

4. Starting price: The starting price of the auction is 350 euros, which suggests that the seller has expectations of a higher hammer price.

5. Market trends: Given that the coin has been auctioned 99 times and has had a high sales percentage (88 out of 99), this indicates a consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 400 to 450 euros. This is based on the fact that the starting price is 350 euros and considering that in the past, coins with similar conditions have reached higher hammer prices.

Therefore, a reasonable maximum purchase price could be 450 euros.