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

1. Condition: The coin has a condition rating of 80 (MS+), which is a good condition and can positively influence its value.

2. Auction History:
- In the past, this coin has been auctioned 3 times and has sold on all occasions.
- Previous hammer prices have been 420, 300, and 260 euros, indicating that interest in the coin is consistent and that it can reach high prices.

3. Starting Price and Average Prices:
- The current starting price is 300 euros, which is above the average starting price of 208.33 euros, suggesting that the seller has high expectations.
- The average hammer price is 326.67 euros, indicating that, on average, similar coins have reached prices close to this value.

4. Recent Trends: In the last auction, the hammer price was 260 euros, which could indicate a slight decrease in interest or in the value of the coin compared to previous auctions.

Taking all these factors into account, an estimated maximum purchase price could be in a range between the average hammer price (326.67 euros) and the highest price reached in the past (420 euros).

Therefore, a reasonable maximum purchase price could be approximately 350-400 euros. This allows for a margin for competition in the auction, considering that interest in the coin remains high.