Parts that are replaceable in a "clevo based" (Sager being one of their rebrand reseller) notebook:
and of course Ram, HDD, and Optical Drive.
Basically, most hardware but NOT THE MOTHERBOARD (Chipset).
There are a lot of resellers selling Clevo rebrands too.
Gentech, AVA direct, digitalstorm, eurocom, cyberpower, xotic, sager... (just these off top of my head) etc.
Some retailers also sells MSI barebones, they are somewhat more noisy than the clevo rebrands but cheaper with less upgradeable paths.