I put my rig together after getting Lonnie's Brutus plans and scouring the world wide interweb for pics and whatnot. I don't weld or fabricate, I'm not real handy and not an engineer and basically had to learn everything as I went, but I'm a quick study. I had never done anything like this before so much of it was learning from scratch.
The upshot is that I outsourced welding the frame but did everything else myself. I now have a modified and kinda-self designed oversized single tier rig with Brutus-style automation that took me about 3 months from start to finish and it works just the way I want. All together, I ended up spending about $1,500 or so less than a B3 or Sabco, but - I went Blichman on the pots, so that's a bit more $ and I also overbuilt the frame somewhat for peace of mind and expandability. My system isn't as automated as a commercial rig and I'm not HERMS or anything but because I put it together, I know I can modify it and built it specifically with the idea of tinkering down the road. Future plans include replacing the burners on the MLT and HLT with electric heat exchangers.
I'd definately recommend building your own if you are not on a tight schedule (When I looked into the B3s they took about 7 weeks to delivery and I think the Sabco's are about the same, by the way). The design and assembly of the rig was EXTREMELY educational and really gets you noodling on your brew process and brewing in general. It forces you to learn and understand the process on a different level. That said, if I move or something and have to sell mine, I'd probably buy a B3 or something and just go about modifying it at this point if I felt the urge, but only because I already built one and figure I basically understand what's going on.
Basically, I'd build one if time isn't a factor and you want to wrap your cranium around the process and geek out a bit. You probably won't really save much money but, like I said, the learning process is pretty valuable.