There just isn't much you can do with the Kilo. Likewise the Seawolf & Carter. At least their propulsors can be cleaned up with a file and made to look more functional. I knocked them out in a day - even allowing for the paint & decals to dry.
Are you committed to small scale? The November by Zveda [sp] in 1:350 scale is pretty good and is more substantial. The Mirage U-Boats in 1:400 scale are nice - and when paired with their etched sets can be spectacular. I picked up a AFV Club Type XXI U-boat in 1:350 last weekend. It is nice too. Then you have the Special Navy 1:72 scale Type XXIII U-boat or one of the big Revell kits.
A nice small, large-scale kit is the Italieri SLC "Pig" manned torpedo in 1:35 scale. It qualifies as a submarine. It is about 8 inches long
If you want to go small & detailed get the Tamiya (ex Skywave/PitRoad) Gato sub in 1:700 and pair it with the Loose Cannon corrected conning tower and Toms Modelworks PE. That makes into a nice piece of work