At the moment mine has a lw, and a stable rug, can use both on colder days and he's doing well with it. I'm going to get a hw this week and thats it, as I do fine as it is with just them.
I would get a LW Rhino (with the liner system) and a couple of Duo liners (100g and 200g). You could then get a MW Rhino (again with liner system) and this would give you lots of options depending on how you combine the rugs and liners. Use the turnouts in the stable and you don't need stable rugs. (Don't forget to check under the rug daily though.)
^ This does, of course, depend on your pony fitting Horseware rugs.
Really depends on the horse, how and where it is kept etc. For example Fany has 2 turnout, 1 stable , 1 fleece for after her bath. Captain has 7 stable, different weights, 8 turnout rugs, 2 sheets, 2 fleeces and a travel rug. (He feels the cold and is in no way pampered!)
My horses live at the top of a fell in Cumbria so weather can be dodgy to say the least. Cappy stays in all winter, his choice- he brings himself in if put out in cold/wet etc. Fany grows a coat like a yak and was turned out last winter in snow naked.
My old Welsh D had the barest minimum of 4 rugs. 1 lightweight turnout, 1 mediumweight turnout, 1 polar fleece and an ancient NZ (from a horse I had in my teens, over 30 years ago) that had 'cross under' straps. He was a big wuss and would feel the cold, luckily he wasn't a rug wrecker! I found I could use the rugs 'in combo', shoving the NZ on top kept it all waterproof so he was okay with outdoor rugs on overnight when he came in.