I believe the wording of the "serpentine" belt was inaccurate. Only three belts should be present in your vehicle: the timing belt, the AC/alternator belt, and the power steering belt. If there is documentation of the serpentine being replaced, it is quite probable that it was the timing belt.
I presume the spark plugs and water pump were replaced concurrently with the timing belt, as they ought to have been. I would replace the plugs if they have not been replaced, and I would note to replace the water pump with the subsequent timing belt change if it has not already been replaced.
The oil cap is not a conclusive location for assessing sludge. If you maintain communication with the previous owner, enquire about the driving habits—primarily short, urban trips or numerous longer journeys of 30-45 minutes (my approximate estimation of what may be adequate) that would have elevated the oil temperature, facilitating moisture evaporation and the combustion of residual fuel. If you remain apprehensive about potential sludge, procure a new front cam cover gasket, remove the front cam cover, and inspect the interior. If sludge is present, remove the intake manifold and rear cam cover to eliminate all sludge. Additional gaskets will be necessary if the rear cam cover is removed.
There may be further items you wish to consider for replacement; but, I would not be concerned about those that appear to be functioning adequately. In the course of a forthcoming timing belt repair, the water pump, tensioner, and idler pulley will be addressed. I have successfully used the original components for 300,000 miles without any indications of breakdown, so there is no need for concern regarding these parts. Additionally, assess the replacement of the front engine seals on both camshafts and the crankshaft during the forthcoming timing belt replacement. The modification is rather simple; simply view a YouTube video on procedures and methods to prevent damage to the new seals during installation.
Examine the hue of your radiator. If it has a yellowish-green tinge, it is likely original and may be deteriorating in brittleness. The plastic undergoes a colour transformation over time.While attempting to detach the oil filter during an oil change, I inadvertently applied pressure to the top radiator hose nipple, resulting in its fracture.
It is an excellent engine. Enjoy!