Wood treatment for summer house
IMHO, the water based stuff should be fine. The walls of a summer house should be protected by the roof and eaves, and shouldn't touch the ground, so they have a relatively easy life (compared to a fence/gate/bench). Oil based products like creosote will smell strongly for months, can leave oily marks if you brush against it, and only comes in one colour. Sometimes that doesn't matter, sometimes it does. Look around your neighbourhood to see what works and what doesn't, as everyone's climate is different.
Interior wood is worth protecting, but you can use something with a nicer finish. The goal here is to fill the pores in the wood so they don't fill with dirt. Modern water-based stains and varnishes are pretty good indoors, or you could use some sort of teak-oil or wax, even a smear of cooking oil will keep it looking nice much longer than untreated wood.
If the preservative says to use two coats, follow the directions. OTOH you do not need to apply a preservative and then a layer waterproof paint. Rot is caused by bugs that need moisture, so applying a poisonous preservative will prevent rot, as would a waterproof paint. The problem with waterproof paint is that cracks and pin holes inevitably allow water through, and once behind the paint, it never dries. Good preservatives shed water but still allow the wood to dry out, much like Goretex fabric.


Probably the most important thing for long life is to keep the walls dry. Maintain the roof and gutters, to keep rain off the walls. Ensure the ground level doesn't get built up, and regularly check that nothing (like bushes, piles of leaves, or other junk) can trap moisture against the walls.