Quote: My mechanic said to stay away from the rover v8's.
Did your mechanic give you any reasoning for this statement?
The Buick/Rover aluminum V8 is a wonderful motor with a terrific heritage and many distinct advantages that include its very light weight, outstanding power-to-weight ratio, and availability of off-the-shelf conversion parts.
Compared to a V6, the Rover V8 has that great, smooth V8 rumble. The engine's relatively short stroke really suits a sports car. This engine characteristically produces a LOT of torque, yet also pulls strongly up past 5000 rpm if you want to go there. (The exact characteristics depend on many variables, of course.)
As more mid-90s Range Rovers are being retired, the newer 3.9L and 4.6L versions get easier and cheaper to find than ever before. (The Triumph TR8 used a 3.5L version of the motor. Think about it: 4.6L is 31% more!)
Here's an excellent overview of the early history of the Rover engine: http://www.britishv8.org/Articles/Rover-Autocar-Article.htm
The "British V8" website contains literally hundreds of technical articles on V8 and V6 engine swaps for MGs and Triumphs - over 5000 photos! - and even an active message board specific to serious performance modifications. Here's the home page: http://www.britishv8.org/
Check out the site's "vendor directory" for shops that specialize in engine swaps and other serious performance enhancements.
You won't have to spend much time on the British V8 site to see that the Rover V8 is especially well loved by engine swappers. There's nothing wrong with most of the V6 options, but I wouldn't dismiss the Rover V8 based on a vague warning from one possibly ill-informed mechanic.
More information on the iron 90-degree Buick V6 that derived from the Buick/Rover aluminum V8 can be found here: http://en.wikipedia.org/wiki/Buick_V6_engine
(Don't confuse this engine with the Chevy 90-degree V6, which is entirely different and not well-suited to sports car use.)
If you're thinking about V6 options, you might also want to check out the 60-degree GM V6 engines because they're narrower and thus generally easier to package:
http://en.wikipedia.org/wiki/GM_60-Degree_V6_engine