Simple questions are good. As long as a simple answer is "OK"

1- The internal oscillators are great for some things. Set it up in the config file *.inc and DEFINE OSC ? at the beginning of the PBP program. If you are using the external MCLR setting then the manual is the way. Some PICs can be set with in internal MCLR, if you do this then you can use that pin as an input and it makes ICSP easier.

2- External oscillators are more accurate. If you are doing critical timing such as SERIAL communication then external is needed.

3-4-5- Go with the three pin oscillators ( you are correct, everything in one package ) I use these
http://www.allelectronics.com/cgi-bi...SONATORS_.html
If the crystal is used then the correct capacitors have to be used (found in data sheet) and the part count goes up and all of that. The three pin setup will go in place of the of the crystal in the manual with the outside pins connected to OSC1 and OSC2. the middle pin goes to the ZERO rail (ground).

For good projects with good explanations look at this site. http://www.rentron.com/pic.htm