whats best spark plug to use and decent price.

The ones Renault use standard are perfectly fine for the job.

I did read something on here a while back suggesting a colder rated plug is better as it produces and even fatter spark than the normal ones supposedly.

If I were you I wouldnt believe all the shpeil about denso iridium blah blah blah.
 
Renault OEM or NGK PFR7Z-TG...

No need to go for anything else.

I wanted Denso/NGK Iridium too (have/had it on all my motorcycles/cars) but there's no need for them because OEM ones are Platinum.