Ok I will try to answer some of your questions. The first thing I wanted to say is that depending on the
dtg machine you get, actually most of them, do print on any color shirt as long as they are equiped with white ink. I myself have the HM1 and I do have white ink in my machine, so I can print on any color shirt.
depending on whether the shirt needs a white ink underbase, the print times vary. I can print a light color shirt in probably 1 1/2 to 2 minutes. With a dark color shirt because I am printing a double layer, white layer and color layer, the time is about double. Plus the dark shirt needs to have a pretreatment applied before printing so the white ink bonds.
The price of the machine depends on the machine you pick. My machine cost was $17,000 and that included 2 sets of inks, the shirt platens and the heat press. I bought mine as a package deal. You can find them used, although I would think this would be harder in the UK, then here in the states, but they are available and might save you some money, if you go that way you want to make sure the machine is in good condition. The ink price also depends on the machine you pick. With my machine because I use a bulk ink system the inks are more affordable then the inks on a closed ink system, that uses cartridges. The bulk ink is about half the price as the cartidge systems.
I am not sure about UK suppliers but I know they are out there

I know that
dtg has several different suppliers in the uk. Well hope this helps some, but I would definately make sure to do really thorough research before jumping in, as it is a big investment.