The point about mobility is important. If you would get any benefit from being able to take your computer out and about with you, then the decision is made at that point.
Also, space might be at a premium, and a laptop can be closed, unplugged and packed away in a few seconds which is not true of a desktop.
Transferring documents should be easy enough - simply copy them onto a removable device such as a memory stick and then paste them on the new machine. Application software (i.e. programmes like Word, Excel etc) will almost certainly need to be reinstalled from their original media. There are some utilities which will attempt to do this for you but, given the age of your existing machine, will almost certainly not prove very effective.