Sonar Professional comes with lots of good synths.
For Bass try the SI Bass, You may have to transpose you midi tracks up 12 ( octave)
For drums you will have Addictive Drums 2 which is Sonars best offering.
You want to change it to GM mode to pay your old drum parts.
You can get by without an audio interface if just working with MIDI, I use asio4all on my midi only rig. If you plan on doing any audio recording for sure you'll need to buy a good audio interface. Most come with midi ports so you can use your old gear still, but once you discover what you can do with the VST instruments you'll probably put those on a shelf.
There is a few ways to insert VST soft synths.
I use the browser on the right, look for the "instrument" icon at the top.
From that drop down you choose the instrument you want.
I use the "First synth audio output" as I don't use simple instrument tracks. I mostly work with my old midi I created on a Atari. So I want to keep them as a plain old midi track.
Once the VST becomes a track you can point any midi tracks output at it. This is the same as you would have been doing to point the track at your midi output device.
Trying the TTS-1 is a good idea as it is a GM synth that plays all 16 channels. It will play any midi file.