In my quest to learn more about grinding, I came across this link. Sounds appropriate given the fact you plan to rent the same equipment from HD. Notice the part where they suggest you buy a hand grinder and disc pads for the edges around the stem wall. Apparently the HD machine won't get those? The article uses Harbor Freight as their source for those tools, which isn't very expensive, but still something you need to plan.
Also, the article talks about renting a shroud (and possibly vacuum) to help control dust, as well as using a TSP after you are done to remove dust. Before I put any chemicals on the floor I would talk with the epoxy manufacturer you plan to use and ask their advise to ensure you get the best bond.
http://allgaragefloors.com/how-to-grind-garage-floor/
Unfortunately the article doesn't dive into time requirements for the work; however, sounds like an all day job according to the guys here. Maybe even 2 days for a shop your size.
We all have different expectations and budgets. I am on the fence myself about what to do as I mentioned earlier. However, one thing I realize is prep is everything with epoxy floors. So if I was going to splurge on something to make sure it was done right, the prep is where it would be. Plus you have to figure out what your own time is worth. Once you know all rental & misc material costs for DIY, then you can compare against a pro quote. If it's $500 more, is 8-16 hours of your own time worth more than that? Can you afford that? Theoretically, if you make $100/hr at work, you'd be better off working and paying a pro. If you make $10/hr at work, you'd probably be better off grinding yourself, assuming you can get the prep right so the life of the final epoxy floor is not compromised.