The Garage Journal has tons of threads on this and several forum sponsors willing to sell you kits, some at a discount for being GJ members. The one thing they will all agree on is prep is the single most important thing, so don't skimp on that. I would stay away from the box store kits IMO.
I'm about to epoxy the floor on my new garage addition now and I just paid a company $900 to come out last Friday and diamond grind my floors (1200 sq ft). Even though it was brand new with no sealer or stains, it was still recommended that they be grinded. I chose to do the epoxy myself and with a tight budget I will be going with a high solids commercial epoxy from Sherwin Williams and a HS urethane coating over the top. Prices will vary from store to store so call around for the best deal.
I did also talk with Greg Weiner from
Armorpoxy as they are one of the sponsors from the GJ forum. Their prices with the GJ discount were very good and they will ship you full kits with EVERYTHING you need to do the job. Unfortunately my project is way over budget already and the SW stuff was the more affordable route.
Hope this helps. I'll post pics of mine when I get done.
Rasky