I have to agree with chopper on this, even on the sound system. Either the buyer won't care that its out of date, in which case you save money and time by not putting in a replacment. Or the buyer will really care about sound system, but will want to make the choice of replacement himself, not have you put in a cheap system.
If you want to do something worthwhile, then wash & wax, throuroughly clean the interior, and even the engine compartment.
You are not in a "spend money to make money" situation. The only exceptions would be if you have unresolved warning lights without an absolutely known/verifiable cause and solution cost.
- Os