My flash kit evolved over a couple of years.
I started with an SB400 as a compact travel strobe and added an SC29 cord so that I could get it off camera.
When I came across David Hobby and Joe McNally I became interested in manual flash so bought a couple of used SB-80DX's on ebay (both immaculate and priced around £90 apiece) to use in manual only.
Initially triggered them from the SB400 by using their built in optical slaves (SU-4) but soon traded up to (used) pocket wizards so that I could fire it all by radio.
Eventually i got an SB900 for some extra grunt and for the option of one fairly powerful TTL flash to use on or off camera.
For indoor use, this means that I have 3 lights that I can can place at will for main, fill and background or rim light.
For outdoors, I put the 900 and the two 80's on a triple bracket into the same umbrella.This gives me just one light but a reasonably powerful one which I usually fire by radio. Three speedlights, a compact stand and a brolly will go in a backpack ....
As an amateur I don't see myself being wealthy enough to afford multiple SB910's to take full advantage of the CLS system but if I was doing paid work it would be completely different because speed would become much more important.
In your position, I would not get a second SB-700.
A new or used SB-900 as main and your existing SB-700 as fill would make sense to me, and I think they would be fine at close range for indoor portraits or for outdoors portraits balanced against evening ambient. Of course you CLS capability with this combination - nice.
If you just want to do indoor portraits, then a studio set will give you most light for the money.
If cash is really limited and you have time to set up, then why not get some cheap generic or older nikon strobes to use in manual mode. You'll figure out exposures quickly enough - no need for a meter unless you need to get it exactly right.
Hope this helps
(edited - hopefully for clarity)