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

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. Auction History:
- The average hammer price for this coin in the past is 673.78 euros.
- The condition of coins auctioned in the past varies, but those with a condition of 90 have reached significantly higher hammer prices (for example, 869.25 euros and 1600 euros).

3. Starting Price: The current starting price is 250 euros, which is relatively low compared to the average hammer price.

4. Sales Trends: Out of the 18 times it has been auctioned, it has sold 16 times, indicating good demand.

5. Comparison with Previous Prices:
- Coins with similar conditions (75-80) have had hammer prices ranging from 240 to 600 euros in recent auctions.

Based on these factors, an estimated maximum purchase price could be in the range of 600 to 800 euros, considering that the condition of 80 is a limiting factor compared to coins in better condition.

Therefore, a reasonable maximum purchase price could be 750 euros.