The 10M is a reasonably well-constructed boat, but I don't think it's considered an offshore boat. It probably beats the Catalina 30 in that regard, and certainly a J24 or J80, but it's not meant as an offshore boat. I just don't really know if it's up to the type of race you are contemplating, as I'm not that familiar with Gulf of Mexico sailing. These events tend to have heavy gear requirements, which might be the biggest problem for such events, as that gear and its installation is neither cheap nor easy to accomplish for smaller boats.
I'd be surprised if you could find a 10M that does NOT have hot water and the other features you want, so I wouldn't worry about that. The one exception is air conditioning; that's probably not a common feature for a 10M in northern climes; no comment about boats that might be found in Florida and Texas (I just don't know).
In terms of sailing as many boats as you can, I agree, that's a difficult proposition. It's a great idea in concept, but the reality is that buyers rarely if ever can go sail 10 different boats they're thinking of buying. IMHO, what is a bit more realistic, and important, is to sail the boat you are thinking of buying, or maybe even the boats on a very short list. For instance, there are enough 10Ms out there that you probably can find a way to get a sail on one by approaching a 10M owner on a dock and explaining the situation (in this economy he might even offer to sell you HIS boat).
Or at minimum, you should be able to sail the actual 10M you are thinking of buying, whether it be on a sea trial or before that. Sellers usually will want a signed contract before you can sail the boat, but there are exceptions. And in any event, just insist that the deal is subject to a sea trial satisfactory to you and that you can walk away if you do not like the way the boat sails (be specific about this point in the contract; it's not an unusual term but it often is not drafted clearly enough).