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

1. Current Grade: The coin has a grade of 85 (MS65/EF40). This is an important factor, as coins in better condition tend to reach higher prices.

2. Previous Auction Prices: By observing previous auction prices for coins with similar grades (85 and 80), we can see that auction prices have varied. For example:
- In recent auctions, coins with a grade of 85 have reached hammer prices of 300 euros.
- Coins with a grade of 80 have had hammer prices ranging from 350 to 380 euros.

3. Starting Price and Average Hammer Price: The current starting price is 400 euros, and the average hammer price for this coin is 294.17 euros. This suggests that the starting price is relatively high compared to the average hammer price.

4. Auction Trends: The coin has been auctioned 31 times, with a sell-through rate of 97% (30 out of 31). This indicates a consistent interest in this coin, which may influence its price.

5. Previous Estimated Prices: Estimated prices in previous auctions for coins with similar grades have reached up to 500 euros.

Based on these factors, an estimated maximum purchase price could be between 400 and 450 euros, considering that the starting price is 400 euros and there are precedents of higher hammer prices for coins in similar conditions. However, given the interest in the coin and its grade, a maximum of 450 euros could be reasonable if one wishes to secure the purchase.