yeah the shower is great place to grow, as far as not understanding hydro its not very hard to figure it out and the total yeild is so much better from hydro. All you need is a bucket with water in it with an air stone at the bottom. You cut a whole in the lid of the bucket and put a net pot with the clay balls in it through the whole. The only expensive investment is your ph and ppm meter, but with this you can keep the fertilizer at the exact levels you want. The thing that i like about hydo compared to dirt is that with this meter you can tell exactly what your fertlizer levels are at. With dirt you have to guess by looking at the plant, and too much fertlizer looks very close to not enough or even other problems. With hydro all you have to do if your plant looks bad is pour out the bucket and add all new water and fertlizer and you can put it at the exact level with the meter. Trying do that with dirt its just impossible to get all that old dirt out of the roots you cant just change your medium if you have a problem. I have grown with both dirt and hydro, i always get atleast double the yield with hydro and its so much easier to keep plants healthy with hyrdo for the reason explained above, not to mention that its easier to keep your room clean and bugs out cause they have no where to hide, and from experience the bugs can sometime come in your dirt.
On another note i like your idea of the shower set up the shower curtain lined with mylar or whatever would work ok but you are still going to have light shineing out of the shower so its not going to be as stealthy as you like, with a grow light they are so bright it is really hard to keep the light from shineing through or come out through the cracks around things your best bet would be to use reflectivex it is a material sold at lowes that is like an insultion for ac ducts and what not it come in diff size rolls the biggest roll ive seen and the best to work with is the 4 foot by 25 foot rolls. It is completely light prove i would get this and some of the silver tape they sale at lowes for ducting(not duct tape) it is like metal tape that on the back side has a layer to pull off kinda like a sticker. i would cover the top of the shower and the front with this. This if done right will help light prove your room so no light shines out and it helps with the helicopters not being able to see your light. they sale zippers at home depot that are for tarps i would put one of these down the front of your room and then you have a sealed light prove room.
yo do need to invest in a good exhaust fan and they sale cheap ones at htg(75.95 for four inch growbright inline fan) you would only need like a 4 inch fan but don't go cheap on this go ahead and buy an inline fan you wont regret it and they are so nice you can completely control your temp with these they hook to dryer hose and you can then vent the air to where ever you want and if smell becomes a problem then this fan is strong enough to use with a small carbon filter which really do work to completely eliminate smell i i wouldn't hesitate to blow my exhaust right into my neighbors house if needed lol( i mean i wouldn't recommend this but you could). You are definitely going to want a good exhaust fan it is just as important as have a the light because during flowering it helps not only control temps but also humidity which if not controlled can ruin a whole crop right at the end of flowering with MOLD, you do not want all that hard work to go to waste. Make sure even if you don't take my advice that you still get some kind of good exhaust system going or you will regret it.
Also if you get your room all sealed like i suggested above (which would be very stealth if you put a shower curtain in front of it after its sealed) You are gonna wanna put some extra work into the intakes so that you don't have light shining out of them i would use two or three 4 inch intakes and buy 4 inch pipe and elbows and paint them black inside then you can have a completely light proof room.
