What are the choices
The engine choice depends really on your budget, Rover V8 which is a good value engine or the chevy which is more expensive but you DO get what you pay for. Then there is the classic Ford engine, you can get the power plants like the 302/351 or the 2ltr pinto.
What are the benefits?
Rover v8- The rover comes with either the carb or Efi, it depends on how much work you are prepared to do installing the engine. Carbs are the easy option and can be installed with the standard s/u - stromberg or the bigger edelbrock, but can still be made powerful with bigger manifolds etc. The Efi however can be more challenging because the running of the engine is controlled by the ecu, but the advantages in my opinion are greater with better fuel economy and tuning capabilities and computer 'chips' which are common for these types of engine.
Free software to test the BHP of your virtual or planned engine: