If part of the commission is to create a permanent artwork, then the artist will need to finish the piece and advise on its installation. Gathering together ideas or artwork from a series of workshops to create the final piece can often be very time-consuming and this should be allowed for in the budget.
As part of your contract with the artist, you should include a maintenance agreement. If the piece is damaged or vandalised, it is good practice to give the artist the first option to repair the piece and to pay them for the repairs. If the piece fails through inherent problems with the design, materials or construction, the artist should be prepared to repair it free of charge up to an agreed cut-off time.
We have copies of sample contracts which cover this type of project. We also have a public art planning pack which gives more detailed information on commissioning a piece of public art.