As far as building goes, try to get fixed site costs. Builders who do house and land packages I think are more willing to do this than if you buy your land separately. The costs for excavation can blow out massivly - as an example, as they dug into our block, they found rock and had to use a form of dynamite to blow it out. Would have been a massive hidden extra for us, but we had fixed costs so the builder had to wear it.

With our house, we did the minimum for electrical work (DH is an electrician, but they wouldn't let him wire up the entire house), because they hugely inflate the prices of additional lights and power points. Maybe get a local sparky to give you a quote for installing a light and a powerpoint so you can compare.

We got everything else done by the builder and in hindsight, we wouldn't do it again. I'm not sure what we would have saved, but we got sick of hearing, "well that's what it is you can't change it". We got them to do our fences (two sides are one style and the back is different ), the driveway (you slip when it's wet), the front garden (we got a yard full of orange crushed rock), carpets and flooring. Needless to say, four years on, our house looks VERY different to what they left us with

We are going to build again, with a builder, but we are going to do bits ourselves.