i've heard the rs1800 head and 2.0 head are the same
not sure really, if you have a 2.0 then use the 2.0 item

as for exhaust go for an off the shelf exhaust such as mongoose or scorpion as they have proven gains and spent loads researching to get the best power from the exhaust, unlike a powerflow system which is just made to fit and wether you gain power or loose it is a gamble

for a 4 branch this aint soo simple, all require minor modding
the choices are
rs2000 4 branch
st170 4-2-1 as they come standard with one
or an ebay jobbie
got a feeling ashley do a 4 branch aswell so depends how big your pockets are

best airfilter
personally k&n panel filter in the original filter housing as they induction kits suck alot of warm engine bay air, unless you sort a good cold air feed

and finally
300mm discs such as st170 brakes will need 16" wheels and bigger

hope this helps