Size to get depends on how big you are. If you are UK size 10 or under you can probably use a size 5 (~4.2m). If you're UK 12-16 then a size 6 (~4.7m). If you're bigger you may need a size 7 (~5.2m). If you're borderline on those sizes, check the actual length of the wrap (some companies do 4.1m/4.6m/5.1m for instance) and consider whether you are long or short in the torso....size up for long, down for short.
Of course if you're 5'1 and size 8 you could in theory use a size 7, but it will be more tricky to get to grips with as you'll have so much extra wrap you don't need.
Reasonable makes to buy cheaply include Colimacon et Cie (also known as Miel et Malice) and Lenny Lamb. Some are pricier than others as they are sought-after limited editions, some makes are just pricier than others. Some are all cotton and some blended with wool/silk/cashmere etc - blends tend to be pricier than all cotton.
If you buy new, some will need more 'breaking in' than others - Storchenwiege and Lenny Lamb need more work than some for instance. Breaking in just means getting it to soften up (think how a new pair of jeans compares to a well worn pair) - using it will help, but steam ironing, tumble drying, sitting on it, plaiting it etc will soften it much more quickly.
Natural Mamas has an 'In Search Of' board - you can always post that you are looking for a first not-too-pricey woven and see what you are offered (you can sense check the price by searching old threads on FSOT or seeing what they go for new).
Most people start out with Front Wrap Cross Carry - there's a good video here If you can get to a sling meet or sling library you can get some useful tips on how to use it too. www.slingmeet.co.uk