redlou the spinning base infant carriers are:
Cybex Cloud Z (and the newer version)
Maxi Cosi Pebble 360
Maxi Cosi Coral 360
Britax Babysafe isize 3
Britax Babysense isize
They're all really safe, really well rated and the toddler seats that go on the bases are also excellent, safe well-rated options too with the option to rear face up to 105cm / 18kg (about 4 years for an average sized child - less for a taller, more for a shorter child). Not a problem at all for safety, although you could improve on safety by buying a rear facing seat which lasts longer, assuming that you would move into a booster after the toddler seat.
But what you describe, only wanting to have the swivel for toddler age is where the salespeople get you! If you bought a normal infant carrier and base, and then buy a separate spinning toddler seat later, what you'll find is that the majority of the spinning toddler seats don't need a base. The base comes attached and you don't need to buy it separately. In fact, there are hardly any toddler seats where you need to buy a separate base. Shockingly, some of the swivel seats that are all in one with base attached, like Joie 360 Spin or even Britax Dualfix are cheaper than the toddler add ons for the swivel base. It's absolute madness! I can't believe how much they charge because basically they know parents are overwhelmed by car seats and instead of looking at the whole selection, they'll go oh I've got a base, let's see what goes on my base, job done, decision made, and they've made an extra £50 out of you compared to if you had bought their (often identical!) seat where the base and seat are permanently attached. AND then you have another baby and the base is in use so you buy another £200 base. Kaching. It's a total con, and I get cross about it.
Yes, the swivel bases tend to only go up to approx 18kg because the isofix fixing is rated for up to 33kg which includes the weight of the child and the seat. Spin seats for toddlers are really heavy and weigh approx 15kg, so it doesn't leave much leeway for the child's weight, even with the newer flexibility about weight limits. If you want to rear face longer than 18kg, which is definitely beneficial especially if you end up with a child on the heavier end of the scale, you'll have to skip the swivel (or buy two seats) because they don't make swivel seats for heavier children. Swivel function is nice, but not the be all and end all. A lot of the larger rear facing seats have really low sides so it's easy to get the children in and strapped in. I have a swivel, and I like it, I think it just depends on your personal preferences - if you're committed to longer term RF, the larger RF seats are much better for that. More leg room for child, less space taken up in the car, and no angst that your child might outgrow the seat at 3-4.5 years and not being sure if they are really ready for a booster. It just gives you the freedom to use it as long as you want and FF when you are ready.