Spark plugs give problems if non NGK ones have been used, most others can
really mess up these engines general performance after a few thousand miles
of use.

Could be HT leads but it's not common.  Only use Nissan leads, they're
better than any others, I took my 10 year old ones off recently to have a
look as my son's Hyundai Coupe HT leads had failed, on his the rubber plug
grip was rock hard and split, on my older higher mileage Nissan ones they
were still supple and as good as new.

None of the 98 Primera sensors are prone to failure really, I would say it's
more likely if the plugs/leads are OK that a sensor could just be dirty,
e.g. the MAF or O2 sensor.  The engine innards could just be clogged up if
the car's been driven slowly on short journeys for years.  For the MAF, it's
either take a chance with careful use of carb cleaner (don't touch the wire,
look it up on Google) or get another from a scrappy as they rarely fail on
the pre 1999 Primeras, for the O2 sensor and injectors plus valves/throttle
body/sticking rings I would try some decent high detergent petrol for a
while like Shell V-Power etc.  My Primera gets slower and slower over a few
months if I use Tesco petrol, even their 99 stuff by the way.  I stick with
Total 95 or 97 or Shell 95 or V-Power as much as possible, Esso seems OK as
well.

Oh, and these engines like to be red-lined once a day to keep in good nick!
If the oil is overfilled even a little they go really sluggish.  Check your
brakes aren't binding, the wheels should be barely tepid after a run with
little brake use.