Oh- no brainer- the vastly improved lighting system in P7 look worth while. IBL and hdri lights and point lights. All must haves, lol, wish I had it. lol
 V2 and V3 have a ton of stuff available still for them free.
The mill girls or the unimesh teens figures also.
I've seen some really good V4 characters, but you'll need the complete package. It's about what? $99.
Miki has allways looked really great too.
And of course, (snickers) anything in my store. :P

Seriously, I'd start with the full V3/M3 packages and the unimesh teens- The morphs and textures work on both. Most bang for the buck. Plus a ton of freebes still around.