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

1. Starting price: 20 euros.
2. Average hammer price in the past: 35 euros.
3. Condition of the coin: 75 (VF), indicating good preservation.
4. Auction history: It has been auctioned once and sold on that occasion.

Given that the coin is in good condition (75) and has had an average hammer price of 35 euros in the past, we can assume that interest in this coin could be similar or even greater in the next auction, especially if there are no other similar coins on the market.

Taking these factors into account, an estimated maximum purchase price could be between the average hammer price and 20-30% more, given that the condition is good and could attract more collectors.

Therefore, an estimated maximum purchase price could be around 45-50 euros. This is based on the good condition and auction history, as well as the possibility that interest in the coin may increase.