Take a rug or large artwork, or patterned upholstery that you have in each room as your inspiration. Take light colours from them for your wall colours.

With out knowing the layout of your home its a little difficult to say, but the top large window section would look great with either roof to floor windows or if $ is ok put on a balcony with bifold doors. Render the brick work and fix the concrete in the front. Garden can make a big difference to the front with out a huge expense

Hi 1st time builder.....best bet would be either hardwood or engineered wood, if you are going down the hard flooring route with your stairs. If your stairs were open......and have potential for noise with kids running up and down, maybe carpet. Another option is vinyl planking, which has had a ton of positive feedback within these Houzz forums. If hard flooring was to be in an upstairs area, I would highly recommend the Karndean Looselay, which has the most acoustic rated backings of any plank. Karndean in Australia will send free samples, so go to their website. Some new builds we are doing here in NZ have hardwood stairs, with matched vinyl planking in other areas, and carpet in bedrooms, although everyones preferences are different!. It will be interesting to get a sample anyway, just so you can compare.

I guess it depends on your budget . I'd tend to initially keep it 'simple' , and do the retaining posts in charcoal , or even a silvery grey . The trim I initially thought charcoal too , but your windows look like they may be white frames , or aluminium , so in that case stick with the white theme too , although the existing garage door/gates in mostly white is then too much white IMO , but you want to change that anyway . Because of the shape , the door is going to be custom -- I'd do a cedar panel type door , or possibly a single piece tilt door may close flush , but I suspect it won't . So clean lines , a clean look to the house , with a darker contrast in the retaining wall . If that doesn't look right , and/or you have a bigger budget , then a new retaining wall in concrete or blocks , with an off-white render , will look classy , but its mixing up eras and styles a bit too much for my liking -- next thing you'll want to square off the garage and all thge windows , and render the whole house . That would work , cost a lot , and look good -- but why ? Apart from the yellow , theres not too much wrong , so embrace it !