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+), indicating that it is in quite 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 price trend seems to be increasing, especially in recent years, where hammer prices of 440 euros and more have been observed for similar conditions.

3. Starting Price and Average Hammer Price: The current starting price is 450 euros, and the average hammer price in the past has been 356.18 euros. This suggests that the starting price is reasonable, but it also indicates that there could be room for the hammer price to be higher, especially if there is interest in the coin.

4. Recent Trends: In the most recent auctions, hammer prices have been higher, suggesting that interest in this coin is increasing.

Based on these factors, an estimated maximum purchase price could be in the range of 450 to 500 euros. This is based on the current condition, previous hammer prices, and the growing trend in interest for coins of this reference. However, it is important to note that the final price will depend on the competition in the auction and the interest from other bidders.