Hi,
I have been an IT contractor for nearly 10 years now, and can bring the folowing to the discussion:
1 - The Ltd. Company is often a requirement if you are working through a recruitment agency - it has to do with the fact that they *can* in some circumstances become liable for your income tax. As has previously been stated on the thread, if your turnover is reasonable, it is also often more tax efficient.
2 - Expenses - I have been advised that if you operate 3/5 or less on a site, then it is not considered your "normal place of work" for revenue purposes. I have never managed to get an official answer on this, but I do personally work to this system. That means that in the situation you describe, you would be entitled to claim mileage at the civil service rates for the 500km pew week, and would be able to claim overnight expenses (either vouched by the Hotel bill, or using the civil service rates). This business of expenses while on site is a constant bug bear for IT contractors. I for example work usually in Meath Cork City and at home in north cork in a week, though depending on how each project is going I might spent 5 days a week in Meath or cork for 6-7 weeks at a time.
One last piece of advice about mileage that was passed on to me - if you are doing very serious mileage (like you are describing) the revenue kind of assume that you would have a company car rather than claim mileage on your own car
Best of luck
Les