Part of the problem that you will have is determining what is currently on the timber.
I quite like Sadolin exterior products but in the data sheet they say "On timber previously treated with creosote or similar water repellent treatments, ensure that they are well
weathered and do not retain any smell or ‘oily’ surface."
http://www.sadolin.co.uk/wp-content/uploads/2012/12/sadolin-timber-stain-technical-data-sheet.pdf
If they aren't weathered enough then you could use a pressure washer.
With regard to colour fastness the waterbased products do last quite a long time. The pigment content is so high that they protect the wood from UV (and thus greying). It is the timber in contact with direct soil etc that i worry about with regard to rotting, a few coats of clear solvent based preservative might help (provided that it doesn't leave a waxy finish).
IMO they do end up looking like they were painted by a diyer who slapped it on a bit thick, ie. you cant see much of the grain any more, but by the other token an opaque oil based finish would not stop the wood from discolouring as the UV would pass through the finish.
A big advantage of the WB products is that you can safely spray them, either using the proprietary DIY systems or an old fashioned pump up pressurised container. With the latter the finish will be quite splattery and you will still need to use a brush to lay the paint off and deal with drips but it is faster than brush alone.
They are also more than capable of hiding darker colours. I have gone from dark blues to lighter greens, albeit with lots of coats (4), something that would have been difficult with the tinted creosote type products.
Do pay head to the weather restrictions though.
