Depends on what you are wanting to listen too, what bands or particular area of listening. Generally though, you are better off with an outside antenna than one of the little dipoles the kits come with.
Strictly for amateur use or are you looking to bring in trunked systems (public safety and the like?) If the latter, what particular system(s) & site(s)? You may need more than one if you intend to use them for trunk tracking
As long as it's a genuine RTL-SDR, Noolec, Airspy, etc. any of them should work fine. The antenna is the big factor in receiving any particular frequencies.
Then it is trying different software programs to find one you like and does what you want.
VHF/UHF or HF? If you are interested in HF then the higher end units like the SDRplay make a huge difference. If UHF trunking the Airspy R2 can do it solo.
While the above people have given good advice, I'm not sure what frequency range you are looking for. That said, I would suggest you read through the below thread, if you haven't. It talks about one persons frustration trying to get frequencies. Some of the info might help you down the road.
I am so upset, as for the past 2 months almost everynight I am up until 3 am trying to recieve hf on my RTL-SDR Donlge. Ive tried everything, and not a single thing has worked. What do I do from here?
VHF/UHF or HF? If you are interested in HF then the higher end units like the SDRplay make a huge difference. If UHF trunking the Airspy R2 can do it solo.