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

1. Current Condition: The coin has a condition of 70 (VF-), which indicates that it is in an acceptable state, but not among the best qualities. This can influence the price, as coins in better condition tend to reach higher prices.

2. Auction History:
- In the past, this coin has been auctioned 4 times and has sold in all of them, indicating a consistent interest in the coin.
- Previous hammer prices have varied, but the highest was 90 euros in an auction in 2020, with a condition of 85.
- In the 2021 auction, the hammer price was 50 euros with a similar condition (85), and in another auction with a condition of 70, the hammer price was 25 euros.

3. Starting Price and Average Prices:
- The current starting price is 20 euros, which is relatively low compared to previous hammer prices.
- The average starting price of 32.5 euros and the average hammer price of 50 euros suggest that there is potential for an increase in the hammer price.

Taking these factors into account, an estimated maximum purchase price could be in the range of 50 to 70 euros, considering that the current condition is 70 and that interest in the coin seems to be consistent. However, since the highest hammer price in the past was 90 euros, you might consider a maximum purchase price of around 70 euros to be competitive, but without exceeding the highest historical value.