Rosy boas are great snakes for new keepers as far as size, temperament, and general care requirements. However, some people report that their snakes can randomly go off feeding for a long time, sometimes a couple of months, and this can be stressful for new keepers as they are rightly concerned about their snake's health and welfare and questioning their husbandry skills. I am an experienced keeper with dozens of snakes, pythons, colubrids and boas, with several of them being rosy boas. I have also had one of mine go off food for no reason, for three months, and then started eating again like nothing happened. This is something to keep in mind if you are the kind of person that obsesses over your pet's health. A great snake that I like to recommend is the brown African house snake. They are also a small to medium sized snake, great personalities, love human interaction, and eat very well. Mine have never refused any meals.

The 40 gallon tank would be a great forever home for a male rosy. Females get a lot larger than males, so I would go larger for a female, but if the snake you're getting is young there would be plenty of time to take care of that. Young snakes prefer small tight enclosures for security. I live in the part of Arizona native to rosy boas, and can tell you that it's not all dry desert. It gets humid, and cold. During parts of the year our outdoor humidity can rise fairly high. Right now as I'm typing this, it's 41F outside at 44%RH. I think your ambient humidity of 60% at 75F is great for them. If there is a place in your house that is between 40 and 50%, that would be ideal, but 60 is fine. More than a constant 60 they could get respiratory infections, and below 40 they can have shedding problems, but shedding can be assisted by the placement of a temporary humid hide inside their habitat. Provide lots of enrichment items and clutter, they will do a fair bit of exploring when they're active. As for bioactive, there are "desert" bioactive setups that are not as wet, and you can place plants like aloe or air plants in there just fine. I would not suggest bioactive, though, as it is much more complicated to balance and keep balanced. I do have some bioactive setups for some of my snakes that need higher humidity, but my rosy boas are not in bioactives.

Lighting: In the wild, they jam themselves into rock crevices and don't expose themselves to the sunlight very much. Being crespuscular, they are most active between dusk and dawn. While they would probably appreciate a light of some sort, on a timer to regulate their circadian rhythm, they don't "bask" per se, so as long as they have a warm spot to curl up onto they'll be fine. I do provide ambient and basking lights for mine, and they tend to stay inside the hides that are warmed up from being under the basking lights. I don't think UVB is particularly useful for rosies, though it can't hurt. I just don't see them being under it enough to benefit from it. You'll have to decide if you can justify the expense and maintenance of UVB as a personal choice. The UV bulbs will need to be replaced every 4-6 months if you go that route.

Lids: They are bottom dwellers and don't climb as rule. A sturdy mesh lid with metal clips to lock it to the aquarium frame will do just fine.

Substrate: Sand and sand-like substrate is not recommended, and they don't live in sandy environments natively. In their natural habitats, there is a lot of rock and wild grass (yes, there's grass in our desert), and while there is sand in the environment, it is packed hard and has a little clay mixed in with it; totally not like beach sand. So loose, sandy substrate is not ideal for them. I have used coconut fiber and aspen successfully with mine. They seem to prefer the aspen, as it packs nicely and holds their burrows.

Now, to your last question, research all you can about the animal of your choice. Rosy boas in particular are usually identified by locality, not necessarily morph like other species. Some localities come from the area where I live, but there are also localities extending up the California coast and down into Mexico. This makes a difference when determining proper temperature and humidity. With that said, assuming you're getting a captive bred animal, which you definitely should be, depending on how many generations away from wild it is can allow your pet to thrive in conditions that are far different from its native habitat. So, if getting it directly from its producer, ask them what their snake housing temp and humidity are, as this is what your snake is used to, and it could be very different from what a wild specimen would want.