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

1. Current Condition: The coin has a condition of 85 (MS+/EF-), which indicates that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction History:
- The average hammer price for similar coins in the past is 334.23 euros.
- In previous auctions, the hammer price has varied, but there are examples of hammer prices reaching up to 470 euros for coins with similar conditions (as seen in the 2019 auction).

3. Starting Price: The current starting price is 400 euros, suggesting that the seller has expectations of a relatively high hammer price.

4. Price Trends: Looking at previous hammer prices, it seems that coins with a condition of 85 have reached hammer prices of up to 450 euros in recent auctions.

Based on these factors, an estimated maximum purchase price could be in the range of 450 to 500 euros, considering that collector interest and the quality of the coin could lead to a higher hammer price. Therefore, a reasonable maximum purchase price could be approximately 480 euros.