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

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction History:
- The average hammer price for this coin in the past is 408.93 euros.
- The last auction with a known hammer price was 380 euros for a coin with the same condition (80).
- In previous auctions, hammer prices have varied, but they have reached up to 750 euros in an auction with a condition of 95.

3. Starting Price: The current starting price is 400 euros, which is a good indicator of the seller's expectations.

4. Price Trends: Observing the hammer prices in previous auctions, it seems that coins with similar conditions have had hammer prices ranging between 260 and 750 euros, depending on demand and other factors.

Given that the coin's condition is 80 and considering the average hammer price and the starting price, an estimated maximum purchase price could be in the range of 400 to 600 euros.

Therefore, a reasonable estimated maximum purchase price could be 550 euros. This is based on the price history and the current condition, suggesting that there is potential for the coin to sell for a price higher than the starting price, but not reaching the highest levels observed in higher conditions.