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

1. Starting price: 18 euros.
2. Coin condition: 80 (MBC+), indicating good preservation.
3. Auction history:
- In 2017, the coin was auctioned with a starting price of 18 euros and sold for 30 euros.
- In another auction in 2017, the starting price was 25 euros, with a final bid of 50 euros.

4. Average final bid price: 53.33 euros, suggesting that, in general, coins of this reference tend to sell for higher prices than the starting price.

Given that the condition is good and considering the previous final bid prices, an estimated maximum purchase price could be positioned between the average final bid price and the highest final bid observed.

Taking all this into account, an estimated maximum purchase price could be around 50-60 euros. This is based on the fact that the coin is in good condition and has shown a bidding history that exceeds the starting price.