1. No trouble at all!
2. No real samples on board Reaper as standard although I guess you will get something out of REASYNTH the onboard synth.
3. The Virtual Keyboard is accessed by View/Virtual Midi Keyboard or ALT B as shortcut. You can click on it or use the QWERTY keys. Not much good for real time playing but fine for your purposes.
4. I am no expert in this sampling malarkey but it seems that while drum samples typically come a "one shot" samples naturally enough, melodic instruments tend to come in collections or libraries which is fine but a bit overkill for your needs.
However there are some one shot piano , brass and strings samples here:
http://theremin.music.uiowa.edu/MIS.html
(Just hover over "musical instruments samples" and you get a choice of pre and post 2012!)