welcome to the forums -
the link joe posted is an excellent resource - as is the search function...
most s4 questions can be answered using those tools...
for me the best place to buy was private party... hopefully its an original owner,
completely stock / unmodified...
being that the B5S4 is getting up in age - theres a HUGE aftermarket / tuning community - so
there are lots of writeups / articles on DIY stuff.
If youve wrenched on ur civic then you should be able to crossover w/ ease
but i was lost the first time i opened the hood & saw the engine (w/ covers etc. on lol)
then i looked a little further & was like "where the hell are the sparkplug wires?"