To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Current starting price: 300 euros.
2. Coin grade: 75 (MBC), indicating a medium-good condition.
3. Auction history:
- In 2016, a coin with a grade of 75 sold for 2034.32 euros.
- In 2023, a coin with a grade of 80 had an estimated price of 1300 euros, but it did not sell.
- In 2024, a coin with a grade of 80 sold for 420 euros.

4. Average prices:
- Average starting price: 696.24 euros.
- Average hammer price: 1227.16 euros.

Given that the grade of the current coin is 75, which is lower than some coins that have sold in the past (80), it is reasonable to adjust the estimated price downward compared to previous hammer prices.

Considering the starting price of 300 euros and the price history, a maximum purchase price could be set in a range between the average hammer price and the hammer price of the coin with a grade of 75.

A conservative calculation could be:

- Average hammer price (1227.16 euros) adjusted for the grade: since the grade is 75, we could estimate 60-70% of this value, which gives us a range of approximately 736.30 to 858.01 euros.

However, since the starting price is 300 euros, a reasonable maximum purchase price could be:

Estimated maximum purchase price: 700 euros.

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