I'd go for floor heat (personally I use head cords, most people would find heat mats easier). It works best, it's safest, cheapest, it's your best option.
Size/age is mostly personal preference. Personally I always prefer hatchlings. I find it satisfying watching them grow up, I feel a better connection with the snake when I have the memories of seeing it as a baby, raising it myelf, etc, rather than it just coming into my life as an adult with an unknown history. Again though, personal preference. Many people prefer a larger animal (I personally don't get it, but hey, when it comes to personal preference I don't have to) and I always find larger animals easier to sell. One other reason I prefer younger animals is that as a newbie you generally have a better chance of getting a high quality animal and as an experienced keeper you'll find it easier to get a high quality animal. People breed snakes and have to sell most of the offspring. When people are selling older animals it's often because there is some sort of a problem with the animal, or because the person realised snakes weren't for them or they weren't good at looking after snakes so they sell an animal which hasn't had the best of care. Sometimes you'll find great animals being sold by a person who happens to be moving abroad unexpectedly etc, or they just happen to have a snake which doesn't fit in with their plans, but it's more common for older snakes to be problem snakes and as a newbie it's more difficult to recognise them.
Great choice for a first snake! My first snakes were also Antaresia (though back them we didn't call them Antaresia) and decades later after working with thousands of snakes from pythons to colubrids, elapids, boas and vipers, I still find Antaresia some of the most enjoyable of all snakes to work with.
If you're interested, I'll be going through the Antaresia in about two weeks and I'll have a few of a range of ages and types available.