would a pre amp be required if this was used as a guitar amp?

also, could the insulation micas for the power transistors be replaced with thermal silicon pads to improve the heat dissipation?

A pre-amp is always required, as the guitar pickup signal is way too low to drive this amp. Furthermore, there are no tone and volume controls.

Silicon pads have WORSE thermal transfer…

would K1803 be a suitable pre amp?

the instructions for the power amp include information for a volume control, and I dont need tone controls

No, you need a dedicated guitar pre-amp such as K4102