To estimate a maximum purchase price for the coin that will be auctioned, we will consider several factors:

1. Condition of the coin: The coin has a condition rating of 80 (MBC+), which indicates that it is in relatively good shape, but not of the highest quality (FDC).

2. Starting price: The starting price is 400 euros.

3. Historical data:
- The average hammer price for similar coins has been 283.35 euros.
- In previous auctions, coins with similar conditions have reached hammer prices that varied, but some examples are:
- A coin with a condition of 85 was sold for 340 euros.
- Another with a condition of 90 was sold for 320 euros.
- A coin with a condition of 95 was sold for 350 euros.

4. Price trends: Observing recent auctions, it seems that prices have been increasing, especially for coins in good condition.

5. Market expectations: Given that there is significant interest in this coin (it has been auctioned 270 times and sold 255 times), this may indicate a steady demand.

Based on these factors, an estimated maximum purchase price could be between 350 and 450 euros, considering that the starting price is 400 euros and that the condition of the coin is good, but not exceptional. This suggests that bidders may be willing to pay a little more than the starting price, but not enough to reach the hammer prices of coins in superior conditions.

Therefore, a reasonable maximum purchase price could be 450 euros.