To estimate a maximum purchase price for the coin in question, let's consider several factors:

1. Current starting price: 1500 euros. This is the initial price set for the auction.
2. Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape. This may positively influence its value.
3. Historical data: In the previous auction, the same coin was offered with a starting price of 600 euros and was sold for 700 euros, with a condition rating of 75. This suggests that the market has valued this coin in a range of 600 to 700 euros in the past.
4. Average starting and final prices: The average starting price is 1050 euros and the average final price is 700 euros. This indicates that, generally, coins of this reference tend to sell for below their starting price.

Given that the condition of the current coin is better than that of the previous auction, this could justify a higher price. However, the starting price of 1500 euros seems quite high compared to historical prices.

Taking all these factors into account, an estimated maximum purchase price could be positioned between the average final price (700 euros) and an upper margin that considers the better condition. We could estimate a maximum purchase price around 900-1000 euros, which would reflect a reasonable value considering the condition and sales history.

Therefore, the estimated maximum purchase price would be approximately 900 euros.