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

1. Current Condition: The coin has a condition rating of 80 (MBC+). This is an important factor, as coins in better condition tend to fetch higher prices.

2. Starting Price and Average Hammer Price: The current starting price is 300 euros, and the average hammer price in previous auctions is 266.15 euros. This suggests that the starting price is reasonable, but it may not sell for above this price.

3. Sales History: The coin has been auctioned 47 times, of which 42 have been successful sales. This indicates good demand and consistent interest in the coin.

4. Previous Prices: Looking at previous hammer prices, we can see that in auctions with similar conditions (around 80), the hammer prices have varied. For example:
- In 2020, a coin with a condition of 80 sold for 330 euros.
- In 2019, another with a condition of 80 sold for 300 euros.
- In 2023, a coin with a condition of 85 sold for 550 euros.

5. Recent Trends: In the most recent auctions, hammer prices have shown an increase, which could indicate an upward trend in the value of this coin.

Based on these factors, an estimated maximum purchase price could be between 330 and 400 euros. This is based on the highest hammer price observed for coins with similar conditions and the upward trend in prices. Therefore, a reasonable maximum purchase price could be 350 euros.