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

1. Current Condition: The coin has a grade of 70 (MS-), indicating that it is in acceptable condition, but not among the best qualities. This may influence the price, as coins in better condition tend to fetch higher prices.

2. Auction History:
- The average hammer price for this coin in the past is 393.33 euros.
- The last auction with a similar grade (70) had a hammer price of 350 euros.
- Overall, hammer prices have varied, but it is noted that coins with a grade of 70 have reached hammer prices between 180 and 500 euros on different occasions.

3. Starting Price and Estimates:
- The current starting price is 300 euros, which is slightly higher than the average starting price of 230.56 euros.
- The estimated price for this auction is 500 euros, suggesting that there are expectations for the coin to reach a higher price.

4. Market Trends: Since the coin has been sold on all previous occasions and considering that there is a scheduled auction, this may indicate ongoing interest in the coin.

Based on these factors, an estimated maximum purchase price could be around 400-450 euros. This is based on the auction price history and the current condition of the coin. However, it is important to keep in mind that collector interest and competition in the auction can also influence the final price.