Is my best bet to get a lawyer and sue? If so, approximately how much
should I expect it to cost? How would it work with the requirement for
him to pay the legal fees?
You would ordinarily either hire a lawyer or bring suit yourself in a limited jurisdiction court (the kind that handles misdemeanor criminal offenses and smaller dollar amounts owed, the exact name of the court differs from state to state). You can always bring suit in the county where the debtor resides. If the loan was not for consumer purposes, you could sue in the place where the loan "was made" or in a forum provided by the loan documents (which does not appear to be present), that might be different from the place where the debtor resides.
Usually, you would want to hire a lawyer with offices not too far from the place where you are bringing suit, as limited jurisdiction courts often require in person appearances.
Normally, a lawyer would ask to be paid up front, with you posting a retainer equal to a significant share of the estimated legal fees, called a retainer against which the lawyer would bill until it was exhausted, and would normally bill on an hourly basis. Fees on the order of $1,500-$5,000 wouldn't be unusual.
One factor that would increase the cost would be the fact that your loan is almost certainly at an illegally high rate of interest, and you and your lawyer would have to examine the relevant law to determine the effect of that on the enforceability of your loan, and the correct amount to claim. In some jurisdictions and circumstances, this might make your entitle loan or at least all interest on it and all fees incurred to collect the debt, uncollectible. In other jurisdictions it might just reduce the amount of interest you could recover.
Without this complication, it might have been a matter you could handle on your own. With this complication, you really need a lawyer.
You could probably not legitimately claim the full $18,000 plus attorney fees and costs.
Some lawyers would take a case like this on a contingent fee basis with you only advancing court costs and out of pocket expenses like process serving charged, but they'd typically do so only if they were confident that they would prevail at trial and if they were also confident that the debtor had the ability to pay. On a one-off basis, a contingent fee percentage of 40%-50% would be more common in this situation that the "usual" one-third contingency rate.
"Reasonable" legal fees would be added to the amount you are owed on the loan if the high rate of interest doesn't invalidate this provision. Amounts recovered for legal fees would be paid to you from which you could repay your lawyer whatever you owed your lawyer. In a contingent fee case, usually hourly based fees are awarded and included in the total amount recovered (once the debtor actually pays) and the lawyer would get a percentage of the total collected regardless of what the amount recovered is supposed to be for.
Another option would be to sell your debt to a debt collection firm which would charge you a small sign up fee and then collect a percentage of the debt recovered. The usurious interest rate involved, however, might discourage them from accepting you as a customer or buying the debt.
If the debtor is not collectible with a job and/or real estate with substantial equity, you probably won't be able to find someone to take the case on a contingent fee basis at all, and will probably have to pay an hourly rate.
Do I have the option to take this up via civil complaints? Or is that
only for people who are in the same state?
It isn't clear what you mean in these questions. A lawsuit is commenced by filing a civil complaint, filed by you or your lawyer in a court, and this can be done even if you don't live in the same state as the debtor, although it may be necessary to file it in the state where the debtor lives.
But, the government won't supply a lawyer to help you collect your debt in the way that it would appoint a prosecutor to bring criminal charges against someone who committed a crime in which you were a victim.