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

1. Current starting price: 350 euros.
2. Coin condition: 80 (MBC+), indicating good preservation.
3. Auction history:
- It has been auctioned 3 times in the past, of which it has sold 2 times.
- The average starting price in previous auctions was 483.33 euros, and the average hammer price was 820.5 euros.
- In the December 2020 auction, the hammer price was 666 euros with similar preservation (80).
- In the March 2020 auction, the hammer price was 975 euros, although the condition is not specified.

4. Price trends: The hammer prices in previous auctions suggest that the coin may reach significantly higher prices than the current starting price.

Given that the average hammer price is 820.5 euros and considering that the condition is good (80), an estimated maximum purchase price could be in a range between the average hammer price and the highest observed hammer price.

Therefore, an estimated maximum purchase price could be approximately 800 to 900 euros. This is based on the price history and the good condition of the coin.