Will the 15L have enough room for gear ie first aid kit, base layors,lunch ect for my little 100k event?
Depends which event!
I’d separate the two bits of your original post out and, personally, get one bag for carting stuff about and training and one for the event itself. I’ve got two Inov8 packs that I use, a Race Pro 22 and a Race Elite 15, which essentially do what you’re talking about. I love them both, very comfy and easy to use. A teammate swears by his OMM 25l, though.
I did the 80km Grand Raid des Pyrenees last year (blog about it here) and I’m going back again this year. Last time round I took the Elite 15 paired with an OMM Trio 4l front pack and two bottle holders. Honestly, it was too much.
If it’s a supported event with regular feed points, you need very little. I dumped a load of excess food that I was carrying at 30km, it was just holding me back. The smaller the pack, the less tempted you are to carry stuff you don’t really need. Put together your mandatory kit, a small amount of food and one or two luxuries – I took my dinky 2gb mp3 player, a compact digital camera and Ted, my mascot – and go to a shop to see what the smallest pack you can get them in is. I’m looking at not more than 12l total this time round, probably a 5-10l backpack with one of the Inov8 2l front packs for the convenience gear, route book, camera etc.