what exactly do you mean with high quality?
i am no expert, but i dont believe that the sound of a synth will be "noticably better" with more expensive components.
As i know, the only parameters that can change are: tolerance, lifetime (especially unter high temperature circumstances), and type.
so as an example: if you use expensive capacitors with low tolerance on a filter modul in your synth, the real cutoff frequency will be nearer to the specified value.
another: if you use high temperature caps in your box your cutoff freq wont change that much within the next year. but overall it wont make it sound "better", in fact you cant even guarantee
the effects i mentioned, since every part in your circuit has tolerances which make every DIY synth sound kinda unique.
Greetings, Nasrudin