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

1. Starting price: 100 euros.
2. Auction history: The coin has been auctioned 18 times, of which it has sold 16 times. This indicates good demand.
3. Previous prices: Previous hammer prices varied significantly, but there are examples of high hammer prices, such as 370 euros for a coin in grade 100.
4. Condition: No information is provided about the condition of this coin, which may complicate the estimation. However, the range of hammer prices for known conditions (70-100) suggests that the price can vary significantly depending on quality.

Given that the starting price is 100 euros and considering the average hammer price of 76.31 euros, as well as the highest hammer prices in the past, an estimated maximum purchase price could be set in a higher range, especially if the coin is in good condition.

Estimated maximum purchase price: Based on the price history and demand, a reasonable maximum purchase price could be between 150 and 200 euros, assuming the coin has an acceptable condition. If it is confirmed that the condition is high, the price could be even higher.