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

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

2. Previous Auction Prices:
- The average hammer price for similar coins has been 548.04 euros.
- In previous auctions, coins with a condition rating of 70 have reached hammer prices of up to 550 euros (as seen in the auction on May 26, 2020).

3. Starting Price: The current starting price is 300 euros, which is relatively low compared to previous hammer prices.

4. Sales Trends: The coin has been auctioned 29 times, of which it has sold 28 times, indicating good demand.

5. Previous Estimates: In past auctions, the estimated price has been significantly higher than the starting price, suggesting that buyers are willing to pay more.

Based on these factors, an estimated maximum purchase price could be in the range of 550 to 600 euros, considering that condition is a key factor and that the average hammer price has been 548.04 euros. Therefore, a reasonable maximum purchase price could be 600 euros.