First off, if you want to start out doing robotics, you should follow this tutorial...http://www.societyofrobots.com/step_by_step_robot.shtml
1. Most beginners use Amtel microcontrollers for their projects. However, PIC microcontrollers are also popular. It does not matter if you do or do not have a serial port. Most programmers (the things you use to program the microcontroller) are USB, and if they aren't, you can easily buy a usb to serial converter for your computer. Check out the link I posted, it has all the software you need. (Free too!!)
2. Yes, building robots is pretty much plug and play. You plug everything in correctly, you do a bit of programming, and it should work. The only parts you'll want to reuse are the microcontrollers, servos and motors, sensors, and stuff like that. Stuff like resistors and capacitors and voltage regulators are generally not considered to be reusable. And they're cheap enough so it doesn't matter.
3. Yes, you can reuse different parts for different projects. An experienced robot builder can build a heck of a robot for almost free because they simply use old parts.
4. No. You need to learn to solder. It's that simple. Soldering is not that hard. Watch some tutorials on youtube. Buy yourself a good soldering iron, some good solder, and it's as simple as putting the soldering iron on one side of the pad, putting the solder on the other, and waiting for the solder to melt. Buying circuit boards ready made will be way too expensive, and you'll probably still need to solder the components in anyway. I don't want to sound mean here, but you won't get very far in robotics without knowing how to solder.
Just follow the $50 robot tutorial, and you'll be good to go. Ask questions on the forums and we'd be glad to help. Welcome to SoR.