I would work backwards from the mods and studio accessories you need.
I assume you want a backdrop, lightstands, a softbox, a beauty dish, some triggers and maybe some strips, reflectors and barn doors.
You will also need grids and maybe flags if the space is small.
You could get all of that stuff for $500 cheap and nasty, $1000 good and solid, or ALL of your money if it's Profoto.
Then you need to work out how many lights you need. I think 2 minimum, but you're better off with 3 or 4.
If you have speedlights these are good enough for hair lights, but may fall short in a strip box.
A big issue with studio lights in a small studio is finding ones that go low enough, the lower they can go the better. I would say you will need them to go down to 18Ws or lower if you want to shoot pretty wide open (f4 or wider). You can use an ND to overcome some of this, but that makes focusing harder and you'll get more misses, especially if you don't have modelling lights.
I think Cheetah Lights might be a good option for you, especially if you want to use them on location as well.