Not stupid question- no question is stupid. Transistor quality, especially output power transistors, affects amp quality in every way possible: distortion, power output, current output, reliability. You can easily check the main chareacteristics of transistors yourself. but you cannot easily check the long-term reliability.
Most items are OK, even Ebay
Many, many good suppliers: Digikey, Mouser, RS, Farnell, CPC, Maplin ...
If you are going to buy from first-line retailers, let me know as soon as possible, because then we can use different components to increse the performance of the amp, without much increase in cost.