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 rating of 20 (BC-), indicating that it is in quite poor condition. This generally negatively affects the hammer price.

2. Current Starting Price and Previous Prices: The current starting price is 170 euros. The average hammer price in previous auctions is 198.53 euros, suggesting that, under more favorable conditions, the coin could reach higher prices.

3. Auction History: In the auction history, it is observed that the coin has had hammer prices that vary significantly, from 102 euros to 700 euros in better condition. However, given that the current condition is quite low, it is unlikely to reach the higher prices.

4. Price Trends: The general trend of hammer prices for coins of similar condition seems to be in the range of 100 to 200 euros, with some peaks in better conditions.

Considering all of this, an estimated maximum purchase price for this coin, taking into account its low condition and price history, could be around 120-150 euros. This is based on the fact that, although the starting price is 170 euros, the condition and price history suggest that a lower price would be more realistic for a buyer.