I think you can very well still have plants in a windowless room- as long as you're willing to give them regular little 'windowsill holidays' in another room. I'd go with succulents, there are some great warm colour combos (see Pinterest for ideas) and the geometric shapes mimic your tiles. Of course, there are some incredible fake ones around too, saw one last week I could not pick from the real thing. I'd be tempted to mix in some of both to balance aesthetics with practicality! Separately, I'd be careful of adding any soft furnishings (rugs, spare towels, etc) to a small powder room. We are all aware these days that a flushed loo spreads germs a meter around it- I wouldn't want to be collecting and trapping them! I think (subtle) fragrance and votives would warm this lovely space without impeding hygiene.

have you personally inspected the property to clarify the available space as the real estate photos show that bed 3 is already being used for the living area so the property is really only two bedrooms plus open plan living dining kitchen and attempting to add an upstairs powder room would mean having the toilet open into the kitchen living area ..it may be possible to install a tiny powder room above the stairs (say $10-$20,000... if you could gain Council approval to do it) but another property with an existing upstairs bathroom would be more comfortable for yourself as well as your guests

If you can’t afford installing extra back splash, then get the tradesman to take out the grey caulking and replace with white caulking. Assuming that the vanity bench top has been installed straight and it is the wall that is the problem. Definitely add more back splash as your first option.

Looks like a version of a floating shelf. One for the vanity and one for the towel shelf. Its framed up and then boxed in. You can find similar DIY on Pintrest via https://pin.it/3hio07M or search DIY floating shelves. Haven’t seen them commercially available in Australia, would need to be a custom job.