First of all you may not need to get internal walls knocked, there is a lot of work than can be done with old buildings apart from knocking them and also the internal walls may be supporting the roof structure but that would need to be checked by someone, a competent builder should be able to tell you this.
Regarding you question about engaging an architects firm to do work, yes they do all the work that you need to get done from doing the planning drawings to engaging a builder to producing construction drawings, giving you a rough estimate of the cost of works etc. The main thing to do is to employ someone who is used to doing the work that you want done.
Regarding the heritage office, try employ a Grade 1 conservation architect to see what you need planning for etc and also they should be able to do all the architectural work.
and ask to speak to Rosanne she should be able to point you in the right direction whether you engage them or not.