Doesn't the OP know that Keyholding is SOOOO last century? In the modern world, you have to move with the times and embrace new trends (But not once they've become popular; do it then, and you're just "begging" the trend like a wannabe). The more talented people are able to look beyond current trends, to what the next trend will be, and launch it before it happens, making it cool.
In this respect, I would like to humbly suggest that the OP reconsiders their Business Plan with urgency; Instead of concentrating on a Keyholding Company, they should consider....
A Key JUGGLING Company! It's soooo unfashonable, that it's about to be it's time to be cool! Instead of rocking up to the location in a company van, they can rock up in style on Unicycles! And those are about to come back in a BIG way!
Why bother keeping a copy of every key for that area in your van, when you can instead give every unicyclist just 3 keys! The fun is finding out whether they're the right ones or not!
For a little extra, you could charge the client for an "Upgraded" service experience! Simply turn up 4 hours after being called, lift up the "Welcome" Mat at their front door you installed, and find a copy of their key!
This is a totally original idea, and i'm trusting you guys to keep it a secret. If the OP nicks my Keyholding Underlings National Team (Barely Against Grouppe) [Or KUNT(BAG) Squad], i'll write him a very, very, very, very angry letter!