Lawnedge and Backdoor Rlwy

As my first foray into the garden (and into railway modelling at all for around 40 years!), I have just bought a job lot of Bachmann E-Z Track, with built-in plastic roadbed and nickel-silver rails. This will be laid in a simple "temporary" ground-level oval (possibly with a passing loop), approximately 2m by 4m, on the decking to educate the Domestic Authorities as to the benefits of running a railway in the garden. Once phase 1 has been achieved, Phase 2 is intended to add a junction from the oval on a raised track along the fence and at places over the flowerbeds, gently sloping to around ground level, over about 8-9m then a loop round a tree (with a lift-up section to allow access to the rear gate) to return up the gradient back towards Phase 1. Phase 3 will involve a junction from the loop to allow a ground-level(-ish) extension along the border to a low-level station at Lawnedge. Phase 4 will be a junction from the oval to terminate somewhere near the Backdoor. I have some Peco flexitrack which I intend using for Phases 2-4, but an alternative may be to use Kato Unitrack, which is similar to the Bachmann eBay purchase, but considered better quality, more expensive, and less available second-hand. Does anybody have experience using these tracks outdoors, and is there anything I can treat them with to reduce the impact of UV from sunlight. Also, how much of a problem is track expansion likely to be, and what can I do to minimise damage?