I don't know how well Techmarines work out of the GK codex, but for a Vanilla SM player, there are so many different variations to play them, model them, and paint them.
Some of my favorite variants are the Master of the Forge with conversion beam, and a Techmarine with Servo Harness and Combi-Melta.
They can probably be outfitted similarly in the GK codex, but you'd have to check to be sure. Good luck with your search!