With such a limited operation is computerising everything really viable? Problem with computers is all the faffing around with backups and the like. Shoving a couple of books in a fireproof safe at the end of the day may well be much more cost effective.
I would consider that you would need to be running half a dozen aircraft to make a computerised system worthwhile. By that time you would probably need to have a full time secretary to deal with all the admin stuff and deal with calls and customer enquiries.