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

1. Condition: The coin has a condition rating of 80 (MS+), which is an acceptable level, but not the highest. This suggests that the price should not be the highest of previous sales.

2. Previous auction prices:
- The average hammer price for coins of similar condition (80) has been 3100 euros.
- In previous auctions, hammer prices have varied, but the range has been between 2000 and 5600 euros, depending on demand and other factors.

3. Current starting price: The starting price is 1600 euros, which is below the average starting price of 2112.5 euros. This could indicate a buying opportunity.

4. Sales trends: The coin has sold in 11 of the last 13 auctions, indicating consistent interest in the coin.

5. Proximity of the auction: There is an auction scheduled in the coming days, which may influence demand and price.

Based on these factors, an estimated maximum purchase price could be in the range of 2500 to 3000 euros. This is based on the fact that, although the starting price is low, the condition and previous hammer prices suggest that the coin could reach a higher price at auction.

Therefore, a reasonable maximum purchase price could be 2800 euros.