I'm wondering what your motivation might be for considering designing and building such a thing yourself. Is this data link a one-off, something that you will do for yourself only and just the once? Or is it something that you want to build as a product for sale to others? I ask because what you are trying to do is not trivial, and you may be much further ahead buying a ready-made RF transceiver module of some sort rather than building something yourself. Even if your goal is simply to learn how to do it, you may still be better off with something modular.
The question of frequency remains, but with off-the-shelf modules at least you have a limited choice. The range that you get will be a function of several key variables:
- the transmit power
- the transmit antenna
- the modulation type and bandwidth (which determines the minimum SNR ratio needed at the receiver to decode correctly)
- the receive antenna
- the location or orientation of the antennas, including height above ground
- the frequency
So, as you can see, even if you must choose a module from a limited selection, a choice that effectively locks in your choice of power, modulation and frequency, at least you can still play with antennas in order to maximize the range.
The regulatory rules dramatically limit what you can do. For example, in most countries there are license-free ISM bands which you can use, but they limit your choice of maximum power and modulation types. For example, here in Canada you can choose 915 MHz but are limited to 1 Watt of power, and only if you use some sort of spread spectrum modulation. That's just an example. Many experimenters choose to use an ISM band. If you are a licensed Radio Amateur, you have many other band choices and powers available. Perhaps you could consider earning an Amateur License as a step to more elaborate experiments.