The pics are a too blurry to read anything, but I think I know what I'm looking at ;)
There should be no reason for this set up not to work - but you might want to think about where you're going to store other equipment once you get more. Since you want to take this trunk to shows it'll probably be pretty packed full.
Like where your brush box is going to go...
How much are you looking to spend? You might be able to buy a trunk for cheaper. After you by supplies and spend all the time to build it.
Here is the trunk I have: KV Pet & Equine / KV HealthLinks - Dog, Cat and Horse Supplies, Pet Meds and Nutritio
It has the brush box and TONS of space in it (I once hid in it during barn hide and seek with the little kids - and I'm 5'9"!). For a one day show you'll definitely be able to fit everything in it, saddle included. When we're at 3 day shows everything fits in it but my saddle(s).