Hmm, it seems you need such a terra former to be near a million tons to be cost effective, that orbital habitat may only cost 200 build points, but the armour for 250 of ship costs over 350 BP, but the armour cost for extra tonnage goes down as your ship increases in size, anyway, terraforming modules cost way more per ton than that one orbital habitat does. My test design at tech tier 1 was about a million tons, went 200km/s with nuclear thermal, had 6 billion kilometer range with 2.5 million litres fuel(360 days travel ), 2 armour layers, 6 CIWS, 1 habitat, cryogenic storage for 50k colonists, and most importantly 25 terraforming modules which could fully terraform an atmosphere less world in 40 years. Total cost was 17k build points, not too bad, can complete it in 3.5 years with starting factories and production tech. Not something I had thought if before, but seems very effective.