Let me set out why I need a windows phone / tablet, then the suggestions would be appreciated.

In short, I have developed a billing solution for Lawyers (Every Minute Counts)
There are many android apps that can be used for certain elements.

However, I have a clock timer app that is in windows. It is already integrated so no need to re invent the wheel.
So where as I would have installed the android apps onto their phones/tablets etc for tracking time out of the office, I can now use an already integrated app that is tried and tested.

So I want to make this available, it necessitates the client having a windows based tablet / phone, which would be minority.
I would probably supply it, so cost should be low.
If they want to use the tablet for other stuff then they can upgrade, but I am working on what I need.
In essence a phone can work, as they are merely using a stopwatch - hence no need for screen.
In this regard Lumia would be the easy pick (I guess).
But If they want to use a tablet then I need an option.

Thanks in advance.