To estimate a maximum purchase price for the coin that will be auctioned, we will consider several factors, including the coin's condition, previous auction prices, and the average auction price.

1. Coin Condition: The coin in question has a condition rating of 80 (VF+). This indicates that it is in relatively good shape, which may positively influence its price.

2. Previous Auction Prices: By looking at previous auction prices for coins with a similar condition (80), we can see that they have reached auction prices that vary considerably. For example, in previous auctions, coins with a condition of 80 have reached auction prices of up to 190 euros.

3. Average Auction Price: The average auction price for this coin is 98.9 euros, which provides a good reference for establishing a maximum price.

4. Starting Price: The starting price of the auction is 18 euros, which is quite low compared to previous auction prices.

5. Market Trends: Since the coin has been auctioned 1,295 times and has had 999 sales, there is an active market for this coin, suggesting that there may be competition in the auction.

Based on these factors, an estimated maximum purchase price could be in the range of 150 to 200 euros, considering that the condition is good and there are precedents for high auction prices for similar coins. However, since the starting price is low, buyers may be willing to bid more, which could bring the final auction price closer to the upper limit of this range.

Therefore, a reasonable maximum purchase price could be 200 euros.