Hiya,
I got my Boss 290 from Walliston Wreckers over here in WA. Probably doesn't help much in itself but the price I paid for the engine and a 5 speed Tremec quick shift gearbox was $4500 which, according to my engine builder mate, was a pretty good price. It's got 70k on it which is still not bad.
I second the notion about trying to get as much with the engine as you can. If you can get the ECU, BEM, ignition etc from the same car, you will be way ahead of the game. Not the end of the world if you do as I have done and purchased a complete wiring loom seperately to the engine but you will need to get the whole lot programmed before it'll do anything
Also with the Boss 290, be wary of the height of the beast. For instance, in mine, I am currently sourcing a new sump (
oil pan) as it is too low when mounted. I may also have to either change out the throttle body or get the bonnet redesigned as the thing is very high
Cheers,
Adam.