LoRaWAN is a tightly defined standard, so any reasonable vendor offering will work with TTN.
There are some differences in performance as there are different chipsets used - for instance the RAK2245 uses Semtech SX1301 chip whilst the Pi0 gateway uses the RAK2246 which uses the SX1308 and the RAK2287 uses the SX1302 chip. You can have fun disappearing down the rabbit hole of Googling which is better and then post your results - get some popcorn and watch the major nerds here get in to a mass debate.
Different vendors come with differences in build quality. But they are too damn complicated to build for them to be knocked up on the cheap.
In terms of community help, browse the Gateways category to see what’s active and the sort of responses. But you should also review the support from the vendor as they should be the subject matter expert, it’s all best endeavours here and we encourage people to search first and ask questions later.
For configuration, vendors typically provide pre-built images for the Pi or some form of installer for a standard Pi image.
All that said, you’ll notice that RAK define all the Pi HATs as developer gateways - they aren’t weather proof and using an SD Card as main storage comes with some inherent risk in terms of wear & tear - which can be mitigated / managed - but consider that the majority of external gateways use Read-only Flash and runs everything in memory. However you can get good results with a branded SD Card, move the logging to RAM and check in on it monthly, a Pi gateway can run ‘forever’.
You will need an appropriate enclosure to put it next to its antenna or use a cable to reach it. The higher the better for the antenna.
If you take a RAK Pi based gateway and can follow instructions, you can go for unboxed to live gateway inside an hour. There isn’t actually that much to do once they are setup, they just sit there, being a gateway. The fun is finding use cases in your community for devices - flood warnings, feed levels, full rubbish bins, road ice warning, signal light failures and so much more.
I’d recommend reading most if not all of the TTN docs, not to memorise them, but so you get good understanding of all the moving parts.