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

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

2. Auction History:
- In the past, the coin has been auctioned 4 times and has sold on all occasions.
- The hammer prices have varied, with a low price of 320 euros (in condition 70) and a high price of 600 euros (in condition 95).
- The average hammer price has been 430 euros.

3. Starting Price and Estimates:
- The current starting price is 300 euros, which is below the average hammer price of 430 euros.
- The estimated price for this auction is 320 euros, which aligns with previous hammer prices for similar conditions.

4. Price Trends:
- The trend shows that as the condition improves, the hammer price also tends to increase. The last auction with a condition of 95 reached 600 euros, indicating that there is a market willing to pay more for coins in better condition.

Based on these factors, an estimated maximum purchase price for the coin with a condition of 80 could be in the range of 400 to 500 euros, considering that the average hammer price is 430 euros and that the condition is slightly lower than coins that have reached higher prices.

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