2nd /3rd flash unit

O.K. so I have a Metz 48 af,which is often use off camera - I would like to have a 2nd (or even a third) flash working as a slave? (back light soft-box you know the sort of thing)or would it be better to have them as ttl?
o.k. people imagine a limited budget what would you add to your kit to support your main flash
think out door portrait, wedding or small event photography

You can get a radio trigger and buy 2 other cheap flashes from ebay,
i find a radio trigger for 10$ with free shipping,
you can buy also a Yongnuo yn-460, yn-462 flash they have a built in photocells and you adjust the power easily, so your metz could be working as a master or can work as a slave in manual mode as a pttl pre flash will be power up your flashes. as fair as I know when you use any flash off camera pttl is not working anyway. I use a old thyristor vivitar flash as a master, and two photocells on the slave flashes and is working well, my second setup is: radio trigger on camera plus 2 radio receivers plus optional 3rd flash with photocell, but I'm don't like them much (as I have cheap ones and they flash on pre flash also) sorry for my English lads,
I believe I help you somehow

The opposite is true, P-TTL works also in wireless mode if the flash and camera support it.

Since you've studio stuff so I assume also the experience you could get all manual flashguns, they basically behave as small studio flashes then.
Instead of the Yongnuo you could also look at the Lumopro lp-120 or the better lp-160 although they cost twice as much as the Yongnuo do...
For triggers I can recommend the Cactus v4, they really do work proper.

i have sigma 530 dg st super, and wireless mode is working somehow,
i use M mode mostly, i had ( flashes, two radio triggers and 3 photocells .
my best is old style hammer flash with rail made by unomat.
it hase 3 diffrent automatic modes and they work fine, 170v is kicking from pc sync cable (shocked me allready twice) so i have to use it with radio, or on analog. i like old flashes you can get them for nothing now as a manual mode use to scare peaople off. my friends use younguo II with good results - i like old russian flashes powered from network (220v) as
they load verry quick - you can get them for free (with zenith or praktica )

im using vivitar as a master as i can bounce light from the celing - i dont like light from popup flash