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

1. Current Condition: The coin has a condition rating of 70 (MBC-), indicating that it is in an acceptable state, but not among the best conditions.

2. Auction History:
- The coin has been auctioned 11 times in the past, of which it has sold 8 times.
- The average hammer price in previous auctions is 36.13 euros.
- Previous hammer prices varied, with a maximum price of 140 euros in a 2020 auction for a coin in condition 85.

3. Starting Prices and Estimates:
- The current starting price is 18 euros, and the average starting price in previous auctions is 24.73 euros.
- The estimated price for the current auction is 36.13 euros, which matches the average hammer price.

4. Price Trends:
- In previous auctions, hammer prices have varied significantly, but the highest recorded price (140 euros) corresponds to a coin in better condition (85).

Given that the current condition of the coin is 70, which is lower than that of the auction that reached 140 euros, and considering the average hammer price of 36.13 euros, an estimated maximum purchase price could be set in a range that reflects its condition and market interest.

Estimated Maximum Purchase Price:
We could consider a maximum purchase price of around 50-60 euros, taking into account that it is a price above the average hammer price, but reasonable given the auction history and current condition. This also allows for a margin for competition in the auction.