To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the current condition of the coin.

1. Current Condition: The coin has a condition of 70 (VF-). This indicates that it is in an acceptable state, but it is not of the highest quality.

2. Previous Auction Prices: By observing the auction prices of coins with a similar condition (around 70), we can see that:
- In previous auctions, coins with a condition of 70 have reached hammer prices of 21, 25, 33, and 86 euros.
- The trend shows that the hammer price can vary, but it generally falls within a range of 20 to 30 euros for similar conditions.

3. Average Hammer Price: The average hammer price for this particular coin is 98.22 euros, but this may be influenced by coins in better condition or by exceptional auctions.

4. Starting Price: The current starting price is 18 euros, which suggests that the seller expects a lower price initially.

5. Estimation: Given that the condition is 70 and considering the previous hammer prices, an estimated maximum purchase price could be in the range of 30 to 50 euros. This is based on the fact that, although the average hammer price is higher, the current condition limits the selling potential.

Therefore, an estimated maximum purchase price for this coin could be 40 euros.