Building a comp is easy, just choose a cpu, fitting coolers, ram, psu, gpu, motherboard, harddrive, the necessary cables(not that many) and the case and done.
Assembly isn't that difficult, i'm always scared to break something but i even short circuited a cpu once and it still runs just fine. Being careful doesn't hurt though.
Hardware is outdated so quickly that it is really hard to be well informed.
IMO most off the shelve computers will handle trading and office work easily, at least running xp.
Hardware reviews are good at
www.tomshardware.com i guess but there are tons of other sites.
I love
www.silentpcreview.com, don't want to spend my day next to a lawnmower.
Selecting the hardware might be a bit tricky because it needs some knowledge which components fit toegether.
Try this
http://www.daileyint.com/build/