An all weather outdoor shower is easily done by locating the mixing valve inside the house and use a coupling shaft to locate the handle on the outside. You must use a standard tub type lower faucet (with the pull up to activate the shower) to allow the system to drain out when the shower is done. You also have to make sure everything slopes in direction of the water flow. We live in Bancroft Ontario so winters are well below freezing and we have been showering outside for 3 winters. Only issue we have is the shower head will freeze so we typically remove it after a shower or shower without it on when really cold. The system has worked well and it helps that nothing special needs to be done by anyone. no inside valves to close etc. Just turn off tap and everything drains.

See the edging around the top of the tile in the shower? Can I do that to an acrylic wall panel? (I hope this picture works?) Eclectic Bathroom by Chestnut Hill Interior Designers & Decorators MANDARINA STUDIO interior design

I really like the look of the separate bath and showers, it seems a little less budget. Plus, there's something kind of worrying about a bath and shower combo - does anyone else get worried about slipping?

Personally, it's better to have a wall between the bathroom and the toilet so no one gets walked in on. Definitely a good idea to keep the bath in case of reselling, although are you planning on going anywhere? If not, get rid of the bath and get a bigger shower I think.