i have the 701hdv head and the 055x legs, and so do a bunch of other members. its a great setup, and would have no problem supporting a slider. i picked up a 2' indislider for 70$ on DVXuser. yes its a cheap friction slider, but a little wd40 and it works great for my purposes.
Sell kit lens (i got 100 on ebay for mine)
Buy a 701hdv head, legs if u need em
Then use this and ur other equipment for a couple months before buying other stuff. You will then see what u truly need.