That does look like a very nice solution Leigh. I had a darkroom in a bathroom that was able to be re-converted pretty easily. Downside was the size made it a bit slow to print in, but I made quite a few lovely prints in it.
Another darkroom was in a bedroom with attached bathroom- this one had black plastic walls in the bedroom which allowed normal access to the bathroom and worked quite well- prints were washed in the bathtub. This was a more permanent set-up, and if the possibility exists for a more permanent set-up I would say go for it.
Don't under-estimate the benefits of having tempered water in the darkroom- I've worked in many darkrooms (currently in the 8th I've built I think) from 'dry' to 'deluxe' and lugging water is a sure way for me to come up with something better to do instead of print.
As to the toxicity (sp?) of chemistry- for most b&w stuff I think 'quite toxic' is a bit strong- you don't want to be drinking the stuff of course, and don't smoke in the darkroom, but even without any ventilation you're not going to keel over. Of my 8 darkrooms only 2 have had ventilation of any kind. Mixing dry chemicals is when you are most likely to run into trouble with getting dosed- I mixed a lot of chemistry outside before ventilation.
That said, I would certainly invest in a good exhaust fan- and if you are building a space within the garage your options may be wider as a fully light tight fan might not be necessary, depending on where it goes.
Fun stuff the planning stages of a new darkroom! One 'rule' I've heard is to lay out the trays you want to use for the biggest prints you think you'll make- then make the sink big enough to fit trays of the next size up. I can comfortably do 1620's in my sink, and 2024's if needed- a bit tight, and requires some shifting of full trays for ease during the fixer remove/toning/rinse stage. Of course the gallery wanted all 20x24's for the last show- when it comes to a sink bigger is better.
