removing the CAT is pretty simple, i have posted the procedure here (https://www.pakwheels.com/forumreply.asp?TopicID=78490&ForumID=27) i believe if would be helpful, check that out, and do leave your comments
you can get it done from any roadside mechanic, do it yourself using his tools or guide him how to do it.
about springs and spacers, i dont think its a good idea to mess with the original suspension, since its extremely good in its stock form, i have got aluminum spacers installed on top of the rear springs but they make squirking noise when depressed, so i am thinking of getting them removed and just be careful with the bumps etc.
about oilchange etc, i think you should get the stuff from some reliable spare parts shop and get them changed at any decent workshop, there is no rocket science, and if you go through some of the pages in this thread, you'd get a better idea of what oil, filters and plugs to use,
i am still using the stock tires, although i think its the time to change them now
high/low beam is adjusted by pulling/pushing the lever on the right side of the wheel, its just like most of the other cars.