I'm the FOH engineer. And is Samplitude good enough?
No you're not - you're someone who'll stand in front of a mixer and twiddle random knobs without knowing what they do...
Anyone with a working set of ears and a brain can learn how to do live sound, just like anyone with a working body and a brain can learn how to fix a car, ski, skydive or fly a plane. But like all of these things, it takes skill and practise to do it - not something you can do instantly. You can no more run FOH sound than you could fly a 747 to Japan, parachute out onto Mount Fuji, ski down the mountain, and fix twenty cars when you get to the bottom!
It doesn't cost a fortune for a sound engineer for an evening. If you've got 500 people and they're paying as little as a dollar/pound each, you can *EASILY* afford it. Do *NOT* short-change your band and your audience by cheaping out, or the gig *WILL* suck slimey green rotting donkey balls through a straw. Hire the venue's guy and the gig will go well, and by shoulder-surfing, helping him set up and asking intelligent questions you'll be a lot more knowledgeable about FOH at the end of the gig.
I'm only a weekend warrior on sound, but I've been doing it for a few years so not completely inexperienced. If I was on this gig, I'd be hiring the venue's soundman to work with me, bcos I don't know the venue's setup and I don't know the gear. Maybe he'd be standing looking over my shoulder most of the gig, and I sincerely hope that's how it'd go - uneventful gigs are what everyone wants! But if something strange happens, you need to be 100% familiar with the gear or the gig will suffer and your audience will not have a good time. If I can't guarantee that myself, it's only professional to get someone on board with the necessary skills/experience.