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 of 80 (MS+). This is an important factor, as coins in better condition tend to reach higher prices.

2. Previous Prices: By observing previous auction prices for coins with similar conditions (80 and 85), we can see that:
- In several auctions, coins with a condition of 80 have reached hammer prices between 594 and 950 euros.
- A coin with a condition of 85 was sold for 900 euros.

3. Average Hammer Price: The average hammer price for this coin is 1136.93 euros, indicating that, in general, coins of this reference tend to sell for higher prices.

4. Starting Price: The current starting price is 450 euros, which is relatively low compared to previous hammer prices.

5. Sales Trends: The coin has been auctioned 20 times, of which 18 have sold, indicating good demand.

Based on these factors, an estimated maximum purchase price could be in the range of 800 to 1000 euros, considering that the condition is 80 and there are precedents for sales in that range. However, given the average hammer price and the trend of the coin, a reasonable maximum purchase price could be approximately 950 euros.