Hoka are fabulous shoes, I would struggle to run in anything else now. The downside can be the fit - they tend to be narrow, although some of the newer models (like the Clifton 3) have a wider forefoot.
As others have said, the Challenger is the trail version of the Clifton. The latest version of it - the Clifton 3 - is just out.
I've found if you use your American shoe size then that works best. So if you look inside the tongue of most shoes there will be a UK, American and European size. For me I take a UK 10, which equates to a US 11. If I order a US 11 then the fit is right.
There are lots of places that stock them now - they were recently bought out by Decker who make Ugg boots, so distribution is improving. I tend to buy online though - normally from ultramarathonrunningstore.com. I've always had great service from them and their returns policy is good. I'm sure if you Google there are lots of others.
In terms of road shoes, the Clifton is a very popular, light shoe. I personally wear the Bondi - I find it a bit more supportive and substantial. The Stinson is the traditional "full" Hoka with maximum cushioning. The Conquest is a heavier, more controlling shoe for over-pronators. I think they have just released some faster racing shoes too, but don't know much about them.
Yes, SD don't do returns (or customer service). Once you've given them your money it's gone. I think Sweatshop are the same now - only credit notes.
As for the shoes, never worn them myself. I've seen alot around at Parkrun and other places, and from a cursory glance they look like "imitation" running shoes - i.e. they've copied the look and appear to have the same features as "real" running shoes. Whether they perform the same I don't know, but would doubt it.
Given shoes are the only piece of equipment we use, I've never really understood the point of saving £20 over the course of 6 months or however long your shoes last. Why not just spend a bit more and enjoy your running ?
Not sure about a replacement shoe, but everyone has to offer you a refund if you order online - I think it's the law for distance selling. I believe you have about a month to return, but goods have to be in their original condition - I.e. don't tags off or wear shoes outside.