The Summit braces are pretty straight forward to fit. As long as you have plenty of clearance to work underneath the car. As for serving a purpose, well, if you track day the car then its well worth it, as the braces stiffen up the chassis. Otherwise there is no real purpose, unless you like driving fast and want a better handling car to go with uprated suspension, or as an extra bit of bling for a show car.
As for the plenum spacer, definitely worth while for that extra bit of torque, but i also have the modified lower collector for your '06' car which does the same job as the spacer but keeps the height at stock levels. You can still add a plenum spacer for even more torque!! The new price of a lower collector is over 500 quid with the VAT from Nissan. The modified version is selling for 290 quid at certain stockists, and i'm selling it for 190 + P&P. These are brand new as well, so really good value for money in my opinion. I'd have it on my car if it wasn't for the fact that it's an'04' and not the rev up version.