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

1. Condition: The coin has a condition rating of 80 (MS+), 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 coin has been auctioned 5 times in the past, of which it has sold 4 times. This indicates a consistent interest in the coin.
- Previous hammer prices are relevant. For example, in the 2020 auction, the coin sold for 1000 euros with a condition of 80. In other auctions, although the condition was slightly better (85), the hammer prices were lower (600 euros in 2021).

3. Average Hammer Price: The average hammer price of 592.5 euros is a good indicator of the current market value.

4. Starting Price and Estimates: The current starting price is 450 euros, and the average starting price is 437.5 euros. However, the estimated price of 1000 euros in the 2020 auction suggests there is potential for a higher price.

5. Recent Trends: In the most recent auctions, hammer prices have been lower (370 euros in March 2023), which could indicate a downward trend or an adjustment in demand.

Taking all these factors into account, an estimated maximum purchase price could be between 800 and 1000 euros, given the auction history and current condition. However, to be more conservative and considering recent trends, a safer maximum purchase price could be around 800 euros.