Plug leads do occasionally break down and moisture can cause the high tension to track to earth, causing a misfire. Coils do also occasionally go bad but it isn't common with denso gear. On my 99 GS 300 each of the 3 coils serves 2 cylinders so a bad coil would affect both and the do usually (but not always) fail entirely. It would be very noticeable. My leads have done 145,000 miles and are still good. My Denso plugs have done 90,000 and are still doing well but it is really time for new ones.
I wouldn't replace plugs with anything other than Denso or NGK (I use NGK for my race engines).
I wouldn't replace the coils unless they were proven to be faulty.
As for the leads....high quality aftermarket silicone leads appear to be as good as OEM and usually a lot less expensive. They must be top quality ones from a reputable manufacturer.
I don't have a distributor cap but OEM would be best.