(Is proud to own ALL the good 3.5 books

Well what makes the most sense to me would be to just start with the basics, and work your way up from there. Space marines for example. we could just make one "Space Marine" class, lower levels being scouts, mid being regular dudes, higher levels being captains, etc. Librarian, techmarine, chaplain, whatever could be prestige classes. choice of feats would sort of determine if youre an assault, tactical, or devistator marine. idk how familiar with the mechanics of
dnd you are, but dreadnoughts might be easy to make if you treat them more like monsters with hit dice and LA. if you have savage species, you know exactly what i mean. as for balance... it all depends on what numbers you assign everything. a bolter could do 50d12 damage if you want, but the nids would need lots of hit points and comparable weapons to challenge your guys. (nerd muscles flexed)