You need to go to a proper climbing suppliers, when it is quiet,
You need to take all the upper kit you will be wearing, fleeces, jackets , waterproofs,
you need some professional guidance,
try different sacks on, with decent weights in them , a decent shop will have bag weights to do this with, and wouldn't sell to you without you going through this process,
try the sack on with gear on and with gear off, it has to be really comfortable for both,
anything that is slightly uncomfortable will be totally unbearable after ten miles,
weight is really important, carry ten ounces thirty mies it feels like a stone,
make sure it has a decent cover sack,
I have quite a few sacks, using a Deuter at the moment mostly, but even when you have a favoured brand, the styles differ enormously, and fit is key, if it a perfect fit, you will have an amazing fun time, and fall in love with the hills, if it's not you will hate the whole experience.
I've been taking groups out for thirty years, it really worth spending time getting kit right, take time, get experience fitting help, it makes a huge difference to the overall experience.
It doesn't have to be more expensive getting the right kit, but never buy a sack without going through the process of elimination but actually trying them all on.
If you are in the south west, go to the "Moorland rambler" bottom of Exeter high street, they offer a fitting service from experts, so will help you get the right sack, then help you adjust your sack so it sits correctly, and teach you to do so, most people do not have their sacks correctly set, it can/will cause injury, and it is much harder work. It takes quite a bit of time to set your sack correctly.