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+), indicating that it is in fairly good shape, although not the best possible condition.

2. Auction History:
- The average hammer price for similar coins in the past has been 4685.71 euros.
- Previous hammer prices for coins with similar conditions (85 and 90) have ranged from 4100 to 6200 euros.

3. Starting Price: The current starting price is 1800 euros, which is significantly lower than previous hammer prices.

4. Price Trends: Since the coin has been sold in all previous auctions, this suggests a steady demand.

Based on this data, an estimated maximum purchase price could be set in a range between the average hammer price and the highest hammer price from previous auctions. Given that the condition is slightly lower than coins that have sold for over 6000 euros, a reasonable maximum price could be:

Estimated maximum purchase price: 5000 euros.

This price reflects a balance between the current condition and the auction price history, considering that the coin has good selling potential.