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 (MBC-). This is an important factor, as coins in better condition tend to reach higher prices.

2. Previous Auction Prices: By looking at previous auction prices for coins with a similar condition (70), we can see that they have reached hammer prices of up to 35 euros in recent auctions. For example, in the auction on 2020-02-12, the hammer price was 35 euros.

3. Average Hammer Price: The average hammer price for this coin is 86.5 euros, but this includes coins of different conditions. Given that the condition of the coin in question is 70, it is likely that the actual hammer price is below this average.

4. Auction Trends: The coin has been auctioned 83 times, of which 79 have been sold. This indicates good demand and consistent interest in the coin.

5. Starting Prices and Estimates: The current starting price is 18 euros, and the average starting price is 42.43 euros. This suggests that the starting price could be a good indicator of what buyers are willing to pay.

Taking all these factors into account, an estimated maximum purchase price for the coin could be around 35-40 euros, considering that this price reflects both the condition and previous hammer prices for similar coins.