I can understand your situation and on the surface it sounds like a good deal. It would pay to do some comparisons on prices of other Suzuki SX4s for sale of similar model, age and odometer readings, however. Trade-in prices are often inflated to help make a sale, especially if the dealer is retaining a good profit margin in the new-vehicle sale.
While your Commodore is at the opposite end of the scale in terms of age versus odometer reading, it may not be an easy vehicle to sell, so don't get too excited about how much money you may have to spend on the boat. While cars may not overly excite you they still need to be fit for purpose, so make sure you take the SX4 for a fairly long drive to try out both the comfort and performance levels.
Suzuki SX4
It's been a steady seller over the years without being a class leader. It carries out all the basic tasks well, is practical and with a 2-litre engine performs above expectations. Make sure that rather large front windscreen pillar doesn't create a blind spot if your travels include a lot of twisty bends. It sits in a very competitive market segment.
Toyota Corolla
The hatch has been the safe bet for years and is often considered the first choice for many.
Prices for a 2010 model can sometimes reflect their popularity, however. Buying from a Toyota dealer can provide some additional sweeteners such as extended warranties, plus service and roadside-assist packages.
Nissan Tiida
The Tiida never really won the hearts of buyers when sold new here but the market has since been flooded with used imports. I would look for a low-mileage 2010 New Zealand-new model with a proven service history as my preferred first option. It may be very bland in looks but mechanically it's sound with lots of usable interior space.
Driven recommends
Your other option is to simply trade the Commodore in on a smaller car. Selling privately can be a stressful experience and although a simple handover of keys may not deliver the same dollar incentive, it provides coverage under the Consumer Guarantees Act if something were to go wrong mechanically a short time after purchase. Plus you don't have to stay at home waiting for buyers to call around when you could be out on the water.