G'day Angelo and welcome to the forum! Re engine a lot of builders are opting for the 6.0L L76 (VZ SS Commodore) or LS2 (VZ HSV) and these can be picked up as crate motors from a number of places (check eBay for starters) starting at around $6000 if you know how to bargain. For the L76 with 6sp manual and all accessories, looms, etc you will be up for about $12,000. Or you might get lucky and find a wreck but I still think you'll need 8 or 9k.
Regarding colour, the more I think about that the more confused I get. There's an awesome Murciaelago dark metallic gray Cobra with ghost stripes on the forum I've linked to below, and I really liked it a few months ago... now I'm swaying back to a classic 'retro' colour such as red or blue. I reckon the Cobra is a fun looking car and gray tones bring it down a bit. That's just me though.
For tons of photos in different colours, check this
link that Les (Rebel1 on this forum) shared the other day.
Cheers,
Paul