I'm a big fan of the Fabulous Small Block Chevy with iron heads and a Turbo 350 Transmission.

Parts are cheap and you can buy them from very basic to totally wild. Prices range from as low as $1,500 for a (New) basic long block and parts tend to be less expensive than other manufacturers. Intake manifold options are all over the place as well, as long as you steer clear of the Tall Port Race Style Heads.

Transmission choices are easier too since the small and big blocks all fit on the same mounts and have the same bell housing bolt pattern. Should you choose to go with a manual transmission at a later date, then it's nice to know that many of bell housings utilize factory hydraulic clutch set ups.

Finally, I don't think it really matters what you put in the car as long as the block hasn't been bored past.030" and is in good working condition.