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

1. Current starting price: 90 euros.
2. Coin condition: 80 (MBC+), indicating good preservation.
3. Auction history:
- In 2016, the coin was auctioned with a starting price of 40 and 50 euros, and it sold for 170 euros on one of those occasions.
- In 2014, the starting price was 120 euros, but there is no information on the final price.
4. Average final price: 170 euros, suggesting that, in general, coins of this reference tend to reach higher prices at auctions.

Given that the coin is in good condition and considering the price history, the previous highest final price (170 euros) is a good indicator. However, the current starting price is 90 euros, which suggests there may be an opportunity to purchase at a lower price.

Taking all these factors into account, an estimated maximum purchase price could be positioned between the current starting price and the average final price. A reasonable range could be:

Estimated maximum purchase price: 150 euros.

This price reflects a balance between the price history, current condition, and starting price.