First step is to survey the landscape, in terms of amplitude v frequency – you’re going to need a filter between the antenna and the SDR. The question is how narrow a filter will you need. You can do that survey with a wideband SDR and an external antenna, or with a spectrum analyzer and an external antenna. I’d use an external antenna that’s as close to what you expect to use as possible.
You can look elsewhere on these forums for for detailed frequency response plots for the FlightAware filter, and other filters such as SAW filters, and some cavity filters. The filter tradeoffs are over cost, insertion loss, and bandwidth. Lowest cost is the Flight Aware filter, also with low insertion loss. Bandwidth? Good, but SAW and cavity filters are sharper.
SAW (surface acoustic wave) filters are a little more expensive, are very sharp in terms of bandwidth, but tend to have higher insertion loss (around 2 - 3 dB).
A cavity filter is the most expensive. A well built and adjusted cavity filter will also have the best performance in terms of bandwidth, and low insertion loss, often under 1 dB.
Applying those frequency response curves to your environment will help you determine how far to go as far as filtering goes.
It will also help you determine how good your feedline needs to be (in terms of leakage for example), and if your SDR (and Pi) needs to be in a metal box or not.
Let us know what you find, and what you do to get an operating ADS-B system – ask questions and we’ll be happy to help as we can.
I hope you decide to experiment, collect data, and make decisions based on that data, and share with us.
Another approach, just as valid, is to decide with your wallet – how much money am I willing to spend on a filter? Get the best for your budget and see how it works.