I've never run soft synths in any of my bands, but I have seen others do it. Here is my advice:
look up some synths typical of the 80s and get soft synths that are based off of or are digital equivalents of synths of that era. Examples would be FM8 (for DX7), Korg Legacy Collections (for Wavestation, MS-20, Polysix, and Mono/Poly), or some of the Arturia synth-specific programs (Prophet V or Jupiter 8V).
Get a hum remover for your laptop power supply (or a power conditioner if this is technically what it does). Otherwise you'll likely get occasional "digital hiss" that is very audible coming from the frequency fluctuations in the power.
Run all sound through a guitar amp (like a Fender Twin) that has lots of headroom. Turn down the Bass knob or use a para EQ to remove the low frequencies that will easily destroy your speakers and simultaneously clash with the bass synth / bass guitar. Try running in stereo if you can find a pair of small amps that are powerful and light as well. If none of the above run through a bass amp and EQ the lows out again.
Have a NICE chorus pedal! The computer soft synth chorus is likely tweakable and MIDI-able, but its nice to have instant foot control of this part of the sound. If you are the one keyboardist in the group then often times both hands will be busy so its an advantage to have control parameters at your feet.
I usually can get by with two keyboards: a digital (Yamaha DX7) and an analog poly (Alpha Juno, JX-3P, or similar) and have most sounds ready to go come gig time. If your rig isn't working its nice to have backup keys or a rack module to rely on at the very least.
And ...all members have long hair!