You might consider a hybrid of the two options you have set out in your question. You could buy a basic kit that contains elements that meet some of your requirements and then supplement such a kit with additional piecemeal purchases. What you don't want to do is buy an ultra-cheap kit in which most if not all of the items aren't up to the task for you nor do you want to buy an expensive "all-in-one" kit that includes many items you don't need.

For instance , the stands that come with a kit might not be quite robust or tall enough to use with your main lights, but they might be perfectly acceptable for use with reflectors or smaller hair lights or low mounted backdrop flashes. If you are going to need stands for those roles anyway, buy the kit but also buy a couple of nicer stands to mount the lights that came with the kit. Then use your SB700 on one of the kit stands to light your backdrops. The nice thing about most light stands is that they almost universally have one of two thread sizes on the tip. Adapters to convert one size to the other are cheap and readily available to allow an almost endless variety of holders to be attached to the top of the stand that will support whatever item you need the stand to support.

A lot depends on whether you decide to use conventional studio flashes or whether you choose to use portable strobes positioned off-camera as your light sources. For a home studio on a tight budget, there is much to be said for the second option. In that case you will probably go piecemeal. There are also many online resources devoted to Do-It-Yourself flash modifiers and stands that you can try on the cheap when using portable strobes. But if you choose a couple of small studio flashes, then buying them with a small kit that only includes items that are needed and usable for your setting could be a good way to save a little.