Hi Yowetan,
I am at the 1/3 mark on our journey to build our house. Am certainly no expert, but since I'm slightly ahead of you on the journey, will share whatever experiences I have.
We were initially deciding between A&A or rebuild. After speaking to several builders, what I gathered is that the cost difference will be about 100k or so (based on land size of 3500sqft). You will have many structural constraits with A&A though. Whereas for rebuilding, you have the liberty to build it to your exact requirement.
As for assessment of a property, most builders/architects can do it. Some do it for free, some do it for a small fee.
What is your expectation of the design of the house. If you want a designer look, you can start shopping for a architect. Don't worry about the builder, builder can come later. But if there is no need for a designer house, some builders offer a All-in-One service. That is they take care of everything from start to finish.... Architectural, Professional Engineer, Surveyor, Construction.... All under one roof. This route is usually cheaper but you won't get a designer house.
Whether you want an architect or a all-in-one builder, my suggestion is for you to go for a shopping spree now. Meet up with as many architects or builders as you can. Coz after each meeting, you will be "wiser". It's a learning process, so the more people you talk to, the more you will understand and you can make better decisions.
The average fee for an architect is about 6%-8% of total building cost. We didn't like this format of calculating the fee, which is the industry practice. Not only is the fee uncertain, there is also incentive for architect to recommend fancy items to up building cost. So we worked out a lump sum fixed fee with our architect.
Some useful websites :
http://www.asiabuilders.com.sg/asiabuilders2011/catalog.aspx?ind_ctry_code=conSG
http://www.boa.gov.sg/cgi-bin/bassrch.cgi
Feel free to ask me more questions. As mentioned, am not a pro... but will be happy to share my experience.
Good Luck!!