Whatever transmitter you buy, you need to put a “Low Pass Filter” also known as an LPF between it and the antenna. Many cheap transmitters have poor filtering or even none at all. The LPF will prevent any harmonics or spurious emissions from the transmitter from causing interference to air traffic control, emergency services, or other radio stations.
The key to long life as a pirate is to operate a technically clean operation, act like you’re not doing anything illegal and don’t interfere with legal stations.
Good luck with the project!