andrewg,
Obviously some strong views on both sides of the buy vs. build. I can only agree there are pros and cons on both sides of the buy vs. build argument.
My recommendation is to define exactly what you want in an "Audit Software". What is it you want? Scheduling of Audits, Assignment of Audits, Creating, assigning and managing Follow-ups and CAPAs, Rescheduling of Audits, etc. Once you have that complete take a look at what's in the marketplace, and if you have the internal skills and resources to build from scratch.
In the market there are a wide range of products which offer a wide range of functionality. Some will have more features than you'll ever want, or need, and others will be too basic. Your work to define what you need at the beginning will reward you in making the proper choice.
In the commercial market, ask for free online demos of products, ask for access to demonstration sites for hands-on introduction, ask about the upfront pricing and ongoing support pricing.
Finally take note, that all software solutions (commercial and in-house developed) will take a certain amount of time at the beginning to configure properly and get "up and running". But once that investment in time is complete whichever method you choose the solution must save you time, increase efficiency, and make your life easier.
If at the outset it doesn't seem like it will do that, then run don't walk away from that internal or commercial software package.
Good Luck!