Monitors… low and medium range, I would suggest multi monitor is probably overkill but high end should be at least dual monitor capable.
I agree with everything you are saying, but that’s because I understand system components/builds… what I’m trying to say is that for the low end PC I would expect “joe average” to think it a bit expensive, and not to really understand much about component quality, or even performance issues… people buy low end PC’s based on price, I would think something like this:
http://www.ebuyer.com/product/227138
is more than enough for the budget buyer, and unless a Linux PC has a similar spec but is cheaper, they’re going to think “but I get Windows with that”, they are not going to understand upgrade paths or component quality, drive throughput, or even frames per second, they are just going to wonder why it’s not cheaper.
Therefore I think you are going to (at least with the low end PC) have to save every penny you can on every component to keep the price as low as possible.
I think budget buyers tend to worry about how “well” it runs after the purchase, and are usually let down but the criteria for the purchase was price, so to please everyone (impossible) you are going to have to build a low end system that is both cheaper than what’s available and performs quite well… OK, starting to get your point… so why bother with budget… but people who buy higher end PC’s tend to have a bit of savvy, at least the ones who are considering Linux (I would think), although I can see some thinking why self-build if the price is right and I’ll get some kind of support/guarantee.
It’s a tough one this…
I don’t know, maybe I don’t give people enough credit 
Again, who are the target… if it’s business then they can be sold the Linux benefits, then price point becomes less important for the low end machine, but if you are thinking of the ‘home’ market…
There certainly seems to be a market for Linux PC’s, at least in the US… it may be an idea to see what people like Zareason, or System76, etc. sell over there. (and then obviously factor in that ALL the components are going to be more expensive over here… as is everything
)
http://linuxpreloaded.com/
I don’t know, just thinking out loud… maybe price points of as close as possible to, say:
£300 - Low end… although hard to see any money in this, except maybe in support.
£500 - Mid range
£800 - High end
and try to build PC’s to suit the price points
Of all the system I’ve built for people over the years, nearly ALL came to me with a price point and said “build the best you can for ‘X’ amount”… even when told the best way of building is “what do you want its capabilities to be”, then I’ll tell you how much, and give you some options.
It was once pointed out to me that when I asked “What do you plan on using it for?”, most people didn’t even understand the question, and just thought “I want it to do everything a normal PC does”, so I was confusing them with the question, and had to keep it simple, like “do you plan on playing the latest games/how much have I got to play with?”.