I've got an Escalera Staircat. They are pricey if you get them brand new but they come up reasonably regularly on Ebay. I got mine for £500 which was fully working but with a fair few dents and scratches. They are built to last though (designed for industrial use) so although it doesn't look pretty, it still performs well. They are also relatively simple - the electrical/control side consists of a car battery, a switch, a circuit breaker and a motor and there is an authorised parts dealer in the UK should you need to replace anything. The only thing I needed to replace was the feet which were a bit worn. I'll never have to change them again though. As Neil says, they aren't fast but that's because they are geared to carry up to 1,200 pounds / 500+kg. They are therefore way over spec for even the heaviest of pinball machines. There are older models that were made for 700 pounds / 300+kg. I've not seen any on Ebay but these might be a bit faster / lighter and still well over spec for a pinball machine if you can find one.
They take a bit of getting used to as you have to compensate for the fact that the truck is going to try to tip away from you as it moves but once you get the hang of it, it's easy enough (and I'm speaking as a short ar$e office worker). The biggest trick I learned is if you need to take a break halfway up/down, don't take a break with the wheels on a step as you will still need to exert effort to stop it moving on it's wheels. Instead, take a break when the truck is between steps supported by it's feet. Sounds counterintuitive but it's easy to balance it in this position with very little effort and it won't slide anywhere. My staircase is an external galvanised one so not disimilar to what it's designed for in a warehouse type environment. I've seen pinball related videos suggesting they work well indoors on wooden or carpeted stairs though. I'd suggest trying to get one of the shorter models for indoor use though as the 72" model is rather tall.