not sure if I should go with 18 or 24" front to back and how tall it should be.
The deeper front to back the better. If you have a choice go with 24" The reason is it will a) give you more water voulme b) give you more surface area c) give you much more flexability in aquacaping
Now. how tall? Just know the taller you go the more light you will need. Dont go taller than 24" for sure.
I would favor glass. Its gonna be a lot heavery to put it into place but it will hold up better and resist scratching.
When desining the wall enclosure remember to give yourself ample room in your closet for the sump and your pumps and your plumbing and lighting equipment. Puting your lighting rig on pullies so it can be rased up out of the way when your working on th tank would be a benifit also.