I've only done 1 event but I have my second lined up and in the process of getting more.
At my first event I used a Dell 610 for uploading, vieing and printing photos but that created a very long queue and I lost sales by people getting bored. I have now added 2 Dell C640s that are networked for viewing via a local intranet JAlbum gallery. I used a hired Mitsubishi 9550 Dye Sub at my last event and am looking buying either one of these or a Olmec OP1000 next week.
I powered at the first event through the mains and hopefully this next event will be mains powered too (I'm speaking to the organiser on Monday) but I am prepared to have to buy a generator in the future for when mains isn't possible. You'll need to buy a good one that has an inverter to provide clean power for the sensitive equipment - normal builders models aren't regulated enough and you risk killing your kit.
Always mount, the cost of the mounts is small in comparison to the selling price but the photos look 10x better. Bare in mind that the costs to the photographer for a 6"x9" is only fractionally more than a 7"x5" but you can sell one for £10-£15 whilst the other might go for £8-£10, I which one I'd rather do when possible.
Be warned though setting up as an Event Photographer is damned expensive and will take quite a few events to recoup your costs. At my last event I took about £500 in sales but made a loss (albeit a small one) as I was renting the printer and it was a long way from home.
EDIT - Always consider having backups of everything - I learned a big lesson at the start of my second day my main laptop died, but as I fortunately had a spare with me so could continue, otherwise it would have been a disaster.