Uncle,
Herr Kaleu is right, the 1/72 U-boat is a perfect place to start...It's big enough (3 ft.), isn't overly expensive ($45) and there's a wide variety of aftermarket parts available for it...Here's what I'm aware of at this point in time, in no order of preference or cost.
White Ensign Photoetched brass detail set
White Ensign Torpedo loading jig
Yankee Modelworks resin pressure hull
Yankee Modelworks Photoetched brass detail set
Nautilus laser cut wooden deck
Steve Nuttall's turned brass detail set (scopes, barrels, and prop shafts)
Warhammer Productions conning tower conversion set
CMK Model torpedo room interior set
Eduard photoetched detail set
Modelbrass photoetched deck
I'm leaving out the figures, since there's a slew of companies doing those.
Hope that helps,
Jeff