Let me know what else I should look for, especially Hantek/Sigilent/Rigol as I only know some decent DMM and generators by them.
Forget Hantek...
Also Peaktech, Owon, Uni-T...
GwInstek I don´t know, but we (work) have several generators and a milli-ohm meter from them, it´s good quality for the money so I couldn´t imagine that their scopes would be crap.
For 300 bucks you wouldn´t get something from Rigol or Siglent, you´ll have to add say 200bucks but then you would get superb scopes from them(for the money).
But be aware you won´t reach a bandwith of 500Mhz, not new, not for this money, not from GWInstek, Siglent or Rigol.
If the 500Mhz are more important to you than "modern" features you could have a look at used old lecroy DSOs.
Waverunner LT364 for example.
But it's like nico said, the old scopes need more attention at regular intervals.