The circumstances in America are different to that of the UK. In America's vast landmass there's a huge variation in climate and many places are considered generally too hot/too cold/too windy etc for guinea pigs to be homed outside and many places have problems with predation. The UK have foxes to be concerned about, the occasional troublesome determined moggy and the occasional dog that has got out. We have adders but usually not close to our homes, unless you live in an adder-rich area with stone walls etc, and even then they are unlikely to target a guinea pig due to the size of the pig in relation to the adder. Basically our snakes are not a problem.
The other thing we have to guard against are rats, who may wish to take the pig's food and cause injury to the pig or damage to the housing as a consequence of getting to the food.
In the USA there's so much more in terms of potential predators and therefore in many places outdoor living is not recommended and outdoor run time should ideally be well supervised in these areas.
Even in the UK guinea pigs suffer in hot and cold weather, and must be protected from both extremes, plus of course wind and rain BUT it's generally quite workable when these things are taken care of.
There are advantages to being outside - one is space. For the average UK home there's often a greater potential for the guinea pigs to have a larger living space outside than inside (but not always).
However, there are advantages in being housed indoors, especially in the main room of the house - the place where most of the human stuff happens. You really do get to know your pigs well, you see them not just at socialisation time but when they do all the other things that piggies do. I favour indoors for various reasons but I appreciate it's not best for everyone, and there are plenty of people who keep happy, enriched and well socialised piggies mainly outdoors, and plenty of folk on here who can advise you on this (as they keep outdoor pigs)
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Ramps you find that in commercially built hutches with more than one story the ramps are very steep - 45 degrees is quite common, and this is off-putting for many pigs. Guinea pigs often prefer a gentle slope, so I'd say don't be put off by having a second floor, but perhaps if there is room to do so make yourself a longer (and therefore gentle) ramp. Even with a two story hutch you need to consider the floor space. It needs to be large enough for all pigs to run around without getting under each other's feet, for them all to share the same floor and still be able to get away from the others when they want to. It needs to be large enough to easily accommodate several enrichment items without causing clutter (tunnels, hidies, soft beds etc).
Cost Sadly large housing doesn't come cheap. However you may find a good bargain second hand, but be careful it's not housed rabbits.
Guinea pigs are not always cheap to maintain. They should be, when all goes well, with the cost of bedding, hay, pellets and veggies being the main things they require. Enrichment toys can be very cheap or even free (there's plenty of threads on this). However it is likely that your pigs will require a visit to the vet at some point during their lives, and although you get pigs that never have to set foot in a vet's surgery there's sadly plenty who do and I will be honest right now - the vet bills can run into hundreds of £'s especially for ongoing issues which pigs can develop.
It's not always easy, but many of us find that keeping a saving pot of money aside for this eventuality helps relive the pressure, and if it's never needed during their life-time then you have the bonus of having saved some money!
to the forum. I'm afraid guinea pigs are not cheap pets ! 
