Threepio is Luke's property and like Han is attached to the Falcon, Luke is attached to Threepio.
Han could have kept the door shut on the droid but for a small bit of added effort he salvaged the droid from certain doom.
That's not the same as deliberately bringing him on a mission in a forest where his skills wouldn't seem to be useful (though they do turn out to be) and where his physical appearance and personality would be an impediment to success.
Alderaan has no weapons, they may buy them on the black market and funnel them to the Alliance but they would have to be hand me downs or in rare cases (like the Tydirium) stolen imperial equipment.
Anyone capable of making fighters would be nationalised by the Empire.
The demand might be there but the supply would get a knock on the door from an Imperial enforcer.
If new ships were available to the alliance they would more likely be Imperial ships which returns to the question of why (if they are cutting edge) the Imperials and Rebels don't fly the same ships?
It seems safe to assume in my opinion that the rebel ships are Clone War salvage hot-rodded beyond their original specifications by a handful of overworked skilled technicians.
Cuban cars.