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

1. Current Grade: The coin has a grade of 80 (MS+), which is quite good. This suggests that its value should be in the upper range of previous auctions with similar grades.

2. Previous Auction Prices: By looking at previous auction prices for coins with grades of 80 and 85, we can see that the auction prices have varied, but generally have remained in a range of 1800 to 3200 euros. For example:
- In 2019, a coin with a grade of 80 sold for 1800 euros.
- In 2022, a coin with a grade of 85 sold for 2800 euros.
- In 2023, a coin with a grade of 90 sold for 4500 euros.

3. Average Auction Price: The average auction price for this coin is 2458.85 euros, which can also serve as a reference.

4. Starting Price: The current starting price is 1800 euros, indicating that the seller has expectations of a final price that could be higher than this.

5. Auction Trends: Since the coin has been auctioned 36 times and has had a high sales percentage (34 out of 36), this suggests there is consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 2500 to 3200 euros, considering that the grade is good and that previous auction prices for similar grades have reached those levels.

Therefore, a reasonable maximum purchase price could be 3200 euros.