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

1. Current Condition: The coin has a condition rating of 70 (MBC-), which is an acceptable level and may influence its value.

2. Auction History:
- The coin has been auctioned 8 times, of which it has sold 7 times. This indicates good demand.
- Previous hammer prices have varied, but the average hammer price is 37.83 euros.
- In past auctions, the hammer price has ranged from 10 to 90 euros, with a highest hammer price of 90 euros in 2016.

3. Starting Price and Average Price:
- The current starting price is 20 euros, which is slightly higher than the average starting price of 17.88 euros.
- The estimated hammer price in previous auctions has been up to 40 euros.

4. Condition of Similar Coins: In the most recent auction of a coin with a condition rating of 80, the hammer price was not recorded, but the starting price was 15 euros, suggesting that coins in better condition can reach higher prices.

Based on this data, an estimated maximum purchase price could be in the range of 40 to 60 euros, considering that the average hammer price has been 37.83 euros and that the condition of the coin is 70.

Therefore, a reasonable maximum purchase price could be 50 euros. This allows for a margin for competition in the auction, given that the history shows prices can vary significantly.