If (and that's a very big if...) you allow players to play Manchines I'd base them around a specific race (most likely human) and most of the time they'd have to play down their strength and dexterity (and hide any weaponry). If they start showing off supernatural strength and hands that turn into metallic blades, then Cloak will come gunning for them if the other players haven't killed him first.

Ability wise either use the stats in the back of Karma or allow stats to an equivalent level to a Stormer (in my opinion a Manchine should be a little tougher than a Stormer, but then this one is unlikely to be the same size as one). Buy the skills and abilities as normal from the 300 points, but be wary of what Advantages and Disadvantages they buy as most will not be practical.

In game time start the player slowly spiraling into madness, testing vs Psychosis every week (2d10+ current ranks in psychosis, 11+ means another +1 to the Psychosis). The Manchine is welcome to spend Experience points negating the increasing psychosis, but would never be able to reduce more than one point per week. As the psychosis starts to dominate the players characters, you'll need to start working out how the rest of the WoP will be affected by this - victims of the Manchine will have families or gangs trying to get revenge, Investigators will start hunting for clues at any murder scenes, and IA will start getting suspicious of the Operatives actions.

The inevitable showdown with the Machine should be carefully orchestrated so that it is the other players who have to hunt the Manchine down, realising that they have been duped all along and that SLA will always look down on them for not identifying it sooner.

Definitely a character with a limited lifespan, but one you can definitely have some fun with while it lasts.