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

1. Current starting price: 350 euros.
2. Condition of the coin: 80 (MBC+), indicating good preservation.
3. Auction history:
- In 2012, the coin was auctioned with a starting price of 90 euros and sold for 140 euros (condition 75).
- In 2013, the starting price was 300 euros and it sold for 480 euros (condition not specified).
- In 2014, the starting price was 280 euros, but the final price was not recorded (condition not specified).

4. Average starting and final prices: The average starting price is 255 euros and the average final price is 310 euros.

5. Price trends: The coin has shown an increase in final prices in previous auctions, especially in 2013, where it sold for 480 euros, suggesting there is significant interest in this coin.

Given that the current condition is better than in previous auctions and considering the starting price of 350 euros, an estimated maximum purchase price could be set above the average final price, but it should also take into account market interest.

Estimated maximum purchase price: It could be reasonable to estimate a maximum purchase price around 450-500 euros, given the price history and the good condition of the coin. This would allow for competitive bidding in the auction without exceeding the market value observed in previous auctions.