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

1. Current Condition: The coin has a condition of 70 (MS-). This is an important factor, as coins in better condition tend to reach higher prices.

2. Auction History:
- The average hammer price for coins with a similar condition (70) has been approximately 20-25 euros in previous auctions.
- In past auctions, hammer prices for coins with a condition of 70 have varied, but there have been hammer prices of up to 32 euros in a 2013 auction.

3. Starting Price and Average Hammer Price:
- The current starting price is 18 euros, and the average hammer price is 26.75 euros. This suggests that there is potential for an increase in the hammer price.

4. Recent Trends: In recent auctions, the hammer price has been around 18-30 euros for coins with similar conditions.

Based on these factors, an estimated maximum purchase price could be between 30 and 35 euros, considering that interest in the coin may vary and that condition is a key factor in its value. However, it is important to keep in mind that the final price will depend on the competition in the auction and the interest of other collectors.