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 (VF+). This is an important factor, as coins in better condition tend to reach higher prices.

2. Previous Auction Prices: By observing previous auction prices, we can see there is considerable variability. For example:
- In 2012, a coin with a condition rating of 90 sold for 300 and 420 euros.
- In 2013, a coin with a condition rating of 85 sold for 463.02 euros.
- In 2014, a coin with a condition rating of 90 sold for 720 euros.
- In 2017, a coin sold for 1500 euros, although the condition was not specified.

3. Average Auction Price: The average auction price for similar coins is 645.58 euros, indicating that, in general, coins of this reference tend to reach high prices.

4. Starting Price: The starting price is 250 euros, which may indicate a reasonable initial price, but not necessarily the final price.

5. Sales Trends: The coin has sold in 15 of the last 18 auctions, suggesting there is consistent interest in this coin.

Based on these factors, especially considering the condition rating of 80 (VF+) and previous auction prices, an estimated maximum purchase price could be in the range of 600 to 800 euros. This is based on the fact that coins in similar conditions have reached auction prices in that range, and given that the condition of this coin is good, it could attract collectors willing to pay a higher price.

Therefore, an estimated maximum purchase price could be 800 euros.