you will have both options , SL and project ,
I will prefer Project , as you can have a project as per your validity of contract of the tower. Also you can use the project till the time you dont close it.
if you create SL then also there wont be any issue but there might be chance of user mistake to entering wrong SL in PO or PR where the SL is not mandatory field.and also there might be chance that you have to create SL based reports to capture expense and other details,
using project you can have PR and PO auto generated once you place material to be procured under that project.and you have some standard report to show alll the required information.
you can check with your project person for this.
Thanks
Kunal