I've seen the ad for the car you are thinking about. It's been listed on craigslist for a couple of months. It was originally priced at $1900, so $1400 is quite a drop in price. It is a wild looking GT with those flames and body modifications. Not what I would want on my car, but it still looks pretty good. I'm sure that all the work must have cost some serious time and money. I was thinking that $1400 was a great price for it especially if, as the ad states, it runs great. However, it does deserve an up close and personal look.
As far as updating and swapping parts with other vehicles, you can look at the forum heading of parts substitutes.
Part Substitutes - Opel Forums. Tips and tricks is another good one.
Opel Tips & Tricks - Opel Forums. If your brother does get it, I hope he becomes a member here. This is a great place to learn and share.
Jerry