Dear friend. I am Joomla Developer. I had much experience about building Joomla component for many version ( Joomla 2.5 + 3.1) If possible you can send me some sample images how it works.
I checked your description and see that we will need to build function for both Backend and Frontend of site support JSON response via basic API to get data from Webservice.
What is my plan
+ Install Joomla 3.1.5
+ Create database diagram base on your requirement ( number of table to save data and config)
+ Build a basic component with the name given by you.
+ Build feature for backend to allow admin add more horse, some event,...
+ Build feature for frontend to show running race, horse info, ...
+ Make Package and send for you as installation package on both 2.5 and 3.1
I am sure about what I will develop and ready to share you updates for each step daily to ensure you follow the progressing of project.
I work full time so we can discuss anytime you want
Tell me if you want to finish your jobs fast and great.
Best regards
Linh