Quality
The answer to your question all depends upon how excellent you want your show to be, and whether or not you can cover the vocals quite well, too. You do not want to play MP3's as Tim suggested. That would be the wrong way to go about it all.
I've been programming and working on being a one-man show for years, all except for a couple of things that I prefer to have a 2nd performer (guitarist who can also help sing).
But to get down to details:
One synth workstation may, or may not, be able to be enough. It all depends on the type of material you are going to do, and the capabilities of your workstation.
For example, my Kurzweil can do phenemonal Hammond B3 sounds, but it uses all its voices to do so, leaving me unable to play other sounds via its MIDI sequencer or even live on other parts of my keyboard. Therefore, I have to use a B3 sample (for some songs is quite sufficient), which frees up the 48 voices to peform the bass, drums, etc. being played by the Kurzweil's Sequencer.
If I am peforming a song with piano (I use all 88 keys for the most part), well then, that does not leave much geography remaining to play the horn parts, or analog synth parts, whatever. Therefore, I need my 2nd keyboard (sitting above my Kurzweil) to play the keys that are programmed for the other parts of a song.
I've found that electric guitar parts are not only difficult to pull off perfectly (believability), but also if it requires a good distortion sound (of which my Kurzweil does have), well, it interferes with how much Effects Units are available for other sounds that the Kurzweil is also playing. Again, having another keyboard is helpful.
The new Kurzweil PC3 is so powerful that you don't run into the issues I have been describing. It has 128 voices, and more than twice as much Effects power for its sounds, and a lot more other stuff, too.
If you cannot sing very well, but intend to do the singing anyway, you should consider using a Vocal harmonizer to add some depth and quality to your singing parts. This requires one of your 16 MIDI channels of your sequencer, dedicated to playing the right harmony notes that your harmonizer will play while you sing.
Drums sounds are very important, so you should take care to find a few excellent drum kits samples to play. Again, the built-in sequencer will handle all this. You do not need to have a laptop computer on stage.
Take note, though, that most built-in sequencers in workstations have limitations somewhere along the line, unlike a laptop and DAW. My old Kurzweil is limited to 64K song files, but I can string them together and get over this limitation, but it takes some careful programming to achieve.
The new Kurzweil PC3 is limited to 110K songs, and I do not know if you can or cannot string them together - which simply means that when the first file is finished playing, the sequencer is told to immediately begin playing the next song file, so that there is no interuption in the entire song process.
If you intend to do folk songs, beatle songs, etc., you don't have to make yourself sound like the whole band. Maybe you just want a drum track, and possibly a bass. It all depends on how realistic you want your overall sound to be.
