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

1. Starting Price: The current starting price is 10 euros.
2. Auction History: The coin has been auctioned 5 times in the past, of which it has sold 4 times. This indicates good demand.
3. Previous Hammer Prices: The previous hammer prices are:
- 40 euros (condition 80)
- 30 euros (condition 70)
- 26 euros (condition 70)
- 40 euros (condition 30)

4. Condition: There is no information about the current condition of the coin, which makes the estimation a bit more uncertain. However, the previous hammer prices show that coins with conditions of 70 and 80 have reached hammer prices of up to 40 euros.

5. Average Hammer Price: The average hammer price of 34 euros is also a good indicator of what could be expected.

Given that there is no information about the current condition, but considering that in the past it has reached hammer prices of up to 40 euros and that the average hammer price is 34 euros, an estimated maximum purchase price could be in a range between 30 and 40 euros.

Therefore, a reasonable estimated maximum purchase price could be 35 euros. This is based on the history of hammer prices and the demand observed in previous auctions.