Need advice. Is $300 a good deal for a 41CV with manuals and 2 expansion cartridges?

[Siegfried]

Looking at recent auction site prices it seems reasonable for a working unit with manuals.

Whether you need and are willing to pay a premium for manuals is a matter of personal preference, some people find digital copies sufficient or even more appealing as they don't need extra space.

Depending on your intentions, the same is true for the Advantage Pac. If you plan to buy/use/play with different programs a Clonix module might be more useful than the Advantage Pac (unless you want 100% authenticity and don't want any non-period equipment).