Power increases can be obtained as you will have lost power through worn dizzy, rotor, plugs etc. I wa snoit saying you will get more power, you will be restoring power from what is lost so will be increasing power.
My valver is on 106k nad has never had a problem with additives, usually STP/Wynns for fuel but now use Forte for oil, coolant, fuel and PAS. If you prefer blocked injectors etc then again just leave alone. They may of course not be blocked. Additives used as and when needed and by someone who knows what they are doing are usually fine. A lot of people argue crap should ramin in engine to keep it from leaking. That to me is like covering cracks hoping tehy dont appear. I prefer preventative maintenance and try to kepp my valver in good condition.
Eon plus are great, they make 6 to fit all cars. They even tell you which are needed on the back. So going to a stockist will help. Its not rocket science, you go to local store or halfords and look. You can even use google. 30 seconds and you find you need Eon 1:
http://www.elcome.ltd.uk/Eon/indexapp.asphttp://www.elcome.ltd.uk/Eon/indexapp.asp
Some people use Bosch super 4, others the standard NGK
GSF is great for getting stuff mail order or direct:
http://www.gsfcarparts.com/http://www.gsfcarparts.com/ Much cheaper for wipers, filters, oils etc.
Haynes recommend Duckham QXR semi synthetic in 14w 40. If you search on here you can read all about oil from a chap called oilman. Think you can actually go lower and higher in viscocity. I have used Mobil 1 Motorosport in 15w 50, Total Quartz 7000 in 10w 40, Castrol RS in 0w 40 and Halfords own. All are great in a Valver. Millers and Silkolene even better so get what you can.
Flushing can be found here:
http://www.williamsclio.co.uk/forum/viewtopic.php?t=5046http://www.williamsclio.co.uk/forum/viewtopic.php?t=5046 or in Haynes manual.
Gearbox oil is simple to check the level, there is aplastic screw at front passenger side that may be caked in muck but that is the fill hole and you fill till it is level with top of hole. Depending on milage may be worth replacing oil rather than mix different types. Funnel and pipe needed to fill and patience as it takes ages.
All this is reasonably simple, just takes a while to find where everything is like gearbox oil filler.
Hope that has helped you more. Buy your own brands you prefer as those I have probably stated are not expensive enough. Oil for example is better replaced often than just have more expensive stuff in there for longer. Halfords isnt bad stuff actually and think the current 2 for 1 which gives you 10 litres for 30 quid is good. Why is another brand better? See oilmans posts and I think Halfords isnt at all bad and better than some.