the kit supplied is abuot £575 and that comes with 25hp jets. jet pairs are £12 a set. you need to start with the 25hp jets to set it up.
as for fitting, to be honest mate as long as you follow the instructions its a very simple job to do yourself, will take 3-4 hours for a novice, i managed 2 hours.
the 3.0 V6 could safely take a 50hp hit, most likely a 75hp hit too but i find the 50hp is more than enough.
as for fitting, best bet is find someone who specialises in tuning as most 'general' garages would possibly be unfamiliar with nitrous.
as i say tho its not a difficult job. no harder than installing an amplifier into the boot really.
daily driving i dont use the nitrous, its normally switched on but i rarely plant it, the acceleration is that strong under nitrous its more for fun than for actual necessety
