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 reach higher prices.

2. Historical Prices: By looking at previous auction prices for coins with similar conditions, we can see that:
- In 2011, a coin with a condition of 90 sold for 750 and 1900 euros.
- In 2012, a coin with a condition of 85 sold for 2700 euros.
- In 2014, a coin with a condition of 90 sold for 18215.46 euros.
- In 2016, a coin with a condition of 80 sold for 337.74 euros.

3. Average Auction Price: The average auction price for this coin is 1395.09 euros, indicating that, in general, coins of this reference tend to sell for relatively high prices.

4. Starting Price: The current starting price is 300 euros, which may indicate a low initial price compared to previous auction prices.

5. Sales Trends: The coin has been auctioned 37 times, of which it has sold 28 times. This suggests that there is a consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be set in a range that considers both the average auction price and the current condition. Given that the condition is 80, which is lower than the higher conditions that have reached significantly high prices, a reasonable maximum price could be:

Estimated maximum purchase price: 800 - 1200 euros.

This range takes into account the current condition and previous auction prices, as well as the interest in the coin.