You don't indicate what the payment structure is with your architect and unfortunately, to have a beautiful architect designed building that suits your aesthetic requirements as well as providing a building that does all you want it to do is expensive - personally I'd be talking to the architect and asking for all options and a full detailed breakdown of how/where this $50K is going and ask him/her what could be reduced and made more cost effective without putting in peril "their vision for your home" (like all business deals - it's a negotiation) - I'd also be trying to ensure that anything that wasn't standard was called out ahead of time so you can try to pinpoint potential budget blow-outs early and maybe make changes early so you're not delaying your build or having added financial stress.

In terms of builders, ask your architect, friends, neighbours (don't be shy about knocking on the door of a clearly newly built home and asking the residents - they'll be flattered and you might find your builder) for recommendations (Truelocal.com.au is also a resource - that's overlooked but can be helpful).

Regardless of how you come across a builder, look for builders that you like as people and with whom can communicate well with and ask them for references and clients that you can physically visit to speak to about: (a) whether they're happy with the work; (b) the scope and style of the work; (c) what out of pocket costs/delays happened plus how problems were dealt with and (d) how the builder interacted with the client on site (you want them to be as gracious, helpful and charming during the build when all of you are under stress as they are now when they want the contract) and regardless of how impressive the recommendations are PLEASE PLEASE PLEASE CHECK their licenses, insurances and affiliations (especially HIA) and ask them for their subcontractors information and check that too - most builders are good people - unfortunately there are a few that aren't - so it's important that you protect yourself. A good builder will expect you to do all of these things and won't be offended - they'll understand that you've got a lot invested in this project (not just money). Hope this helps.