There are cage calculators online to give you an idea of square footage per rat. You should always get at least 2 rats or more as they are social creatures and will get depressed without a friend. The wire spacing should be less than an inch. Half an inch is ideal, but those are harder to find. If you can't find small enough bar spacing you can put chicken wire around it. We did this when they were babies because we had a Houdini. The cage is the big expense. You can get creative with decorations and toys. A lot of Dollar Tree stuff works great. They get all of my holey socks and cardboard boxes.
The Critter Nation is the top of the line. I have no idea why it's so expensive right now on Amazon. It's usually around $250. https://www.amazon.com/MidWest-Critter-162-leak-Proof-Measures/dp/B001NJ0DAY/ref=mp_s_a_1_3?crid=38UEPCPF64YZ&keywords=critter+nation+cage&qid=1637602941&sprefix=vritter+nation%2Caps
I have a similar cage by Prevue and it was literally half the price when I got it. I wonder if it's a temporary mark up due to the holidays or inflated due to the shipping issues. https://www.amazon.com/gp/aw/d/B000QFMYWQ/ref=ya_aw_od_
8 lbs for a twin, 16 lbs for a full/queen, <6 lbs for a medium sized floor mat/dog bed
The best part is you can feel the weight. My sleep is much less restless under one of these and they don’t make you feel like your baking under your covers. Technically it’s not yarn, it’s poly-fill in a cotton tube. I’ve seen videos where you can make your own, but I don’t possess that patience: clootess Arm Knitting Yarn Chunky... https://www.amazon.com/dp/B0897HZK29?ref=ppx_pop_mob_ap_share
Oh yeah, you totally can. Knitpicks sells just the different lengths of cables cables both through their website and on Amazon. They’re a different color when sold individually but I have the interchangeable set with the purple cables and the needles fit the green cable just fine.
Bernat 'Mega bulky' is pretty awesome with 15mm/size 19 needles.
https://www.amazon.com/gp/product/B074Y24FXG/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1