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

1. Current Condition: The coin has a condition rating of 85 (EF-), indicating 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 381.34 euros.
- In previous auctions, we have seen hammer prices that varied significantly, but some relevant examples are:
- A coin with a condition of 85 was sold for 650 euros in November 2020.
- Another coin with a condition of 80 sold for 240 euros in November 2020.
- In October 2022, a coin with a condition of 80 was hammered at 700 euros.

3. Starting Price: The starting price of 300 euros is a good indicator, but the hammer price is usually significantly higher.

4. Recent Trends: Recent auctions show that coins with similar conditions have reached hammer prices exceeding 600 euros.

Based on these factors, an estimated maximum purchase price for the coin in question could be between 600 and 700 euros, as this reflects both the historical hammer prices for coins of similar condition and current market trends.

Therefore, a reasonable maximum purchase price could be 700 euros.