As far as legality goes, Indiana does not have a usury limit of interest, but even if so that applies more to loans than invoices so it would be a stretch for someone to prove it.
As with many thing, the issue will come down to DISCLOSURE. Was the customer aware of the charge at the time they entered in to the agreement? Also, is the charge reasonable?
Reasonable will include what your costs are for the customers non payment, what the market rates are for unpaid invoices and are you trying to gain excessive profits. (The legal system is written so only attorneys are supposed to get profits.)
You will typically need to file a notice of intent before placing a lien on the property.
You do not typically need to hire an attorney to place a lien on property, the best source of information on this would be the county assessors office.
As far as making someone pay, that comes down to how much is it worth to you.
You can sue the debtor, obtain a judgment and try to execute on the judgment, but this will have varying degrees of success.
Your options change regarding collections if the work was commercial rather then residential.
Something else you may want to consider is placing the account with a collection agency. Though they will take a % (20-30ish usually) they can be very effective at getting you paid.
A typical service industry late/finance charge I have seen has been about 18% APR (1.5% per month), or a flat fee of $10.00 per month for billing and collections.