I have a Super Blue model 156. It has power and to spare for my engines and power-hungry Aristo coaches. My son’s layout was powered by a Tech II 2500 HO transformer. We found though, that two trains could be run only if the smoke and lights were shut off on the coaches. This wasn’t acceptable, so I broke down and bought the Blue. It serves my needs beautifully. There is even some ability to walk around with the controller. In fact, the transformer remains in the house whilst the controller is outside with the operator. The Blue was a bit pricey, but money well spent.
There are often any number of transformers on e-bay all the time. It shouldn't be difficult to find a decent transformer for a small outdoor layout. Just remember, it's always best, outside anyway, to have more than one power connection to your track.