Couple of quick answers...
Unless you're riding somewhere super muddy on treaded tyres I can't see that there will be such an appreciable amount of muck on your tyres that you'd ruin your house. I've been using my regular outdoors tyre on my turbo in the kitchen and have yet to ruin the cupboards. That being said, getting a turbo tyre is probably a good idea, as then you won't wear out a decent outdoor tyre. As you've suggested, pick up a cheap spare wheel (or blag one off someone with lots of spare bits

) and a spare tube and away you go. Probably worth getting a spare cassette as well, because there's no point saving the effort of changing tyres to ride your turbo if instead you have to swap your cassette over, which is a ball-achingly annoying job at the best of times.
Sounds like a good bet. Fluid is better than magnetic, but it costs more. Go figure. For a decent fluid one you're looking at £200+ compared to £100+ for magnetic.