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

1. Current Condition: The coin has a condition of 80 (MS+), which indicates that it is in relatively good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Historical Prices: By looking at previous auction prices for coins with similar conditions, we can see that:
- In past auctions, coins with a condition of 80 have reached hammer prices of up to 440 euros (as seen in the auction on 2017-10-18).
- The trend of hammer prices for conditions ranging from 80 to 90 has been between 250 and 440 euros.

3. Starting Price and Average Hammer Price: The current starting price is 350 euros, and the average hammer price in the past has been 351.13 euros. This suggests that the starting price is reasonable and that there is a good chance it will be met or exceeded in the auction.

4. Recent Trends: In the most recent auctions, hammer prices have ranged from 440 to 700 euros for similar conditions, indicating an increase in interest and value for these coins.

Based on these factors, an estimated maximum purchase price for this coin could be between 400 and 500 euros, considering that the condition is good and there is a price history to support this estimate. However, given the starting price of 350 euros, bidders may be willing to pay a bit more, especially if there is interest in the coin.

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