Finding the carbs should be easy enough, every ones idea of a reasonable price is different, think around the hundred pound mark anything less is a bonus, aftermarket carbs for these are more for tuning and cost loads
You can set up your carbs to run K&N's, different size jets are all over the internet
How incomplete is your bike, you will find most of these classics need work even when you spend a few grand
There are loads of 750 owners on here , you should be able to get all the info and know how to do your bike up, its easy if you take your time
Pete