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

1. Current price and previous prices: The current starting price is 3500 euros, which is slightly higher than the average starting price of 3457.14 euros. However, the average hammer price in previous auctions is 5491.67 euros, indicating that, in general, coins of this reference tend to sell for a price higher than the starting price.

2. Auction history: The coin has been auctioned 7 times, of which it has sold 6 times. This suggests that there is a consistent interest in the coin and that it is likely to sell this time as well.

3. Condition: There is no information about the condition of this particular coin, but in previous auctions, coins with a condition rating of 80 to 85 have had hammer prices ranging from 3200 to 8750 euros. This suggests that the condition can have a significant impact on the final price.

4. Price trends: Looking at previous hammer prices, the lowest was 3200 euros and the highest was 8750 euros. This indicates a wide range, but also suggests that, in general, prices tend to be higher than the starting price.

Based on these factors, an estimated maximum purchase price could be in the range of 6000 to 7500 euros, considering that the average hammer price is 5491.67 euros and that coins of similar condition have reached higher prices in the past.

Therefore, a reasonable maximum purchase price could be 7000 euros.