I'd go for the rescue cat, and I have two Pedigree's I paid stupid sums of money for, all because a) finding a rescue near us who would rehome to us was pretty much impossible and b) my DP was reluctant to rescue, don't ask why.
With two cats, related or unrelated, you need multiple litter boxes, and if they are indoor you need to keep up the scooping on those boxes which isn't easy when you work full time. You will come home to the house smelling of shit, trust me.
They will also need lots of space to get away from each other. Whilst cats do love company, they also love their solitude, and need the ability to get away from each other or there will be fights.
Feeding two cats will cost a lot more. My two cost me about £50-80 a month (I'm a bit precious with their food) and ragdolls are big cats, they need a lot of feeding.
They will also cost more to insure. However, just something to bear in mind with the older cat, you might find getting insurance for the older cat difficult and costly due to its age.
As I say, I'd go for the rescue if I was able to, we unfortunately weren't (I got in touch with 12 different shelters, including the big ones, and none of them would rehome to someone who works full time and doesn't intend to allow the cats outside).
However IF you do get a kitten, please please please make sure you do your research and don't buy the first cute ragdoll kitten you see advertised. Make sure you find a breeder who tests for HCM and PK-Def, and one who owns mum and lets you see mum, makes sure the house is clean, ask for proof of the health tests and make sure the mum isn't too young or too old. Buying kittens and puppies is a minefield, contrary to popular beleif there ARE good breeders out there, breeders who make very little profit but do it for the love of the breed, but they are very few and far between.