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 of 70 (MS-), indicating that it is in an acceptable state, but not of the highest quality.

2. Previous Auction Prices:
- The average hammer price for coins of similar condition (70) has been approximately 65-110 euros in previous auctions.
- In the most recent auctions, the hammer price has been around 85-110 euros.

3. Current Starting Price: The starting price is 50 euros, which is relatively low compared to previous hammer prices.

4. Sales Trends: The coin has been sold in 23 out of 25 auctions, indicating good demand.

5. Average Hammer Price: The average hammer price for this coin has been 86.78 euros, which can serve as a good indicator of market value.

Considering all these factors, an estimated maximum purchase price could be in the range of 90 to 120 euros, taking into account that the condition is acceptable and there is a trend of increasing prices in recent auctions.

Therefore, a reasonable maximum purchase price could be 100 euros.