帮我看看我的英语文章有没有错误好吗?

帮我看看我的英语文章有没有错误好吗?
Dear Mr.Gates,
Thank you for your letter which concern about reinstalling and upgrading our system.
We have to,however,finalise some details in advance.
I’m afraid that the date you planned is unavailable because of our originally planned agenda.Your rescheduling would be helpful for us and I want to know exactly how long does the installment take?Furthermore,we require the engineer to install the system on weekend so that it will no influence our daily work.
In addition,we need to keep both of the systems in our company to insure the safety for a month.
I also want to know the engineer’s name and number in order to exchange information efficiently.
Look forward to your reply.
Yours sincerely,
Greg Cai