To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Current Condition: The coin has a condition rating of 80 (MS+). This is an important factor, as coins in better condition tend to reach higher prices.

2. Previous Prices: Looking at previous auction prices for coins with similar conditions:
- In past auctions, coins with a condition of 80 have reached hammer prices of up to 360 euros.
- Coins with a condition of 90 have reached hammer prices of up to 400 euros.

3. Starting Price and Average Hammer Price: The current starting price is 400 euros, and the average hammer price for this coin is 284.7 euros. This suggests that the market is willing to pay more than the average price, especially if the condition is good.

4. Recent Trends: In the most recent auctions, hammer prices have shown an upward trend, especially for coins in good condition.

Based on these factors, an estimated maximum purchase price for this coin could be between 360 and 400 euros, considering that the starting price is 400 euros and there is interest in the coin. However, since the average hammer price is 284.7 euros, a more conservative purchase price could be around 350 euros, to ensure a good buy without exceeding the market value.

Therefore, a reasonable estimated maximum purchase price could be 350 euros.