There are no "essential" rugs. My native pony doesn't own a rug and is 100% happy. What rugs you need, depends on the type of horse you own and the weather conditions in your part of the world.
We have one of those Amigo lite weight turnout rugs for this time of year, had it 2 years its been invaluable use it loads thro the spring and summer even though he is roughed off thro spring and summer, hes got it on today a slight chill in the air today, also when we had those horrific floods 2 summers ago it never leaked at all. Ours is dark blue, think the colour of the this year is red.
Oh yes my mare isnt in at night (im trying this whole leave them out lark and she seems much happier!) but id probably have her in a med underrug and stable rug...hmm
i just bought a super weathabeta combo good price and extremely quick delivery! SpeedyEquestrian.co.uk....i had never heard of them before!
My horse is out today in a light weight Amigo turnout and when he is in in the daytime at the moment he has an Amigo Medium weight insulator as they are quite light weight (if that makes sense, only 150g) but at night he still wears a heavy weight stable rug but with no neck as it still drops quite chilly at night. Hopefully a bit more sunshine and I can get the fly rugs on and a summer sheet at night. I cant wait.
Depending on your horse then at this time of year (and any time of year) rugs aren't necessary full stop. Many horses of all types live outside 24/7 365 days a year rugless.
FWIW my 7/8th TB trace clipped horse in light work is out 24/7 rugless at the moment, he has worn just 1 heavy weight NZ at the coldest point of winter and has come into spring looking very well.
My youngster is clipped out so wears either a light NZ or medium NZ overnight depending on weather and has had the odd day naked in good weather.
The only rug I would consider essential at this time of year is a sweetitch rug if your horse suffers from this. Or a rain sheet if you are lazy and don't want to groom off mud!