Look at what your competitors did and why they failed. There's been two or three businesses like this in Germany in the 2000s - and I think they were not too successful (cannot remember their names).
Mostly, because preinstalled with Windows and then installing yourself is still cheaper than buying from a small, single trader.
I know it's a tough market, but if other small electronics & pc shops can manage to survive it, perhaps I can make it too.
I believe those missed the point about Linux. They thought the rage was all about saving the costs of Windows licenses, and they could not survive the aggressive licensing program of the late Windows XP/Vista period, when Microsoft practically gave away licenses almost for free. I don't believe it's about costs. IMHO it's more about freedom.
Mostly, because preinstalled with Windows and then installing yourself is still cheaper than buying from a small, single trader.