Please keep in mind this is your space, doesn't impact my life at all, and my statements are just some guy talking on the internet. I can be very opinionated.
I'm going to start with some questions that always pop into my mind first, and do matter...
I am assuming this is a dedicated theater room.
What are the room dimensions? Those of us with a more audio-centric mentality (which is not me) may make some more, and frankly better, suggestions for audio than I will. I like good audio, but I'm more practical and don't strive for absolute best sound, but like great value sound for the money. There are some who can spend all of your money and give you better audio than the Monoprice models. There are also those who HATE in-wall speakers, so you gotta pick your poison with those guys.
If the room is large enough, pushing it out a bit, or working behind it will full size tower speakers with a AT screen is a great option. I've done that in a few rooms over the years and it is spectacular for sound. I have used Stewart, but one of my favorites are the models from Seymour AV. They are a fair bit more money than the surprisingly well reviewed models from Silver Ticket. I have used Silver Ticket screens with JVC projectors and they look shockingly good. Proving a good room plus a good projector is far more impactful than a great room and a great screen with a weak projector.
If given the choice between in-walls and towers, I would do towers, and I wouldn't necessarily go with MP speakers. Monoprice is great, but there are some very good and well priced options out there.
I will say, that if you aren't doing the full installation yourself, you may be limited on who you can use for the installation work, and you should expect, at some level, to pay some markup on whatever you get. It's how these places stay in business. Still, you can go in with some speakers, or cut out a 'huge expense' from the system and they will still be happy. Labor is a category that I think is often under-valued in these installs and they make up for it with high margins on equipment sold, which doesn't have as much actual value to you.
So, besides room dimensions, which people here can give you better numbers and options on for filling the room properly and with great quality...
What projector do you intend to use? I'm assuming a JVC, because not JVC is - questionable. Unless you go up to a Christie Griffin. (3-DLP awesomeness)
With a 12'-18' viewing distance, I would consider a 150" diagonal 16:9 screen the sweet spot for size. Front row viewers get excellent immersion for a truly theater-like feel, while back row viewers won't feel so far away from the screen . It also gives enough size without overwhelming most projectors on the market. You don't want the image to be overly dim. I think in the next 5-10 years we will see more projectors come to the market with better peak brightness to help with HDR content, but right now, they aren't as bright as the world would like them to be. But, they are brighter than they were a decade ago.
I will add this now...
Theater colors matter a great deal Dark carpet, dark walls, dark ceiling - GET IT RIGHT! So many really good looking home theaters feature 'light colors', which doesn't match up to any movie theater you've been to in your life. A splash of color is great. But, if you want to add something cool, do so with LED lighting which can be powered off, and won't detract from the experience. Be aware of dark fabric wall coverings like Adamantanium ( http://cine4home.de/test-und-know-how-special-schwarze-heimkino-optimierungen-welche-adamantium-stoffe-eignen-sich-am-besten/
) which is something I would give serous thought to for best image quality.
Get your lights figured out properly. If you are building a 'dark theater', then to light it up like your family room, which IS something you will want to do at some point, you will want DOUBLE the number of lights of a typical room that is painted white with white ceilings and light colored carpet. You want one row of spotlights over each row of seating, plus general room lighting. Spotlights allow you to turn some lights on in the room for eating, or even reading, while having minimal impact on the screen. Then there should be 'general' room lighting using typical BR30 style flood lights. These will light the room from corner to corner allowing easy entry and exit.
Because the room soaks up light, doubling the room lighting allows for you to crank them up and really see what's going on, but most of the time they will be dimmed to about 50% or so for general entry/exit.
I have some photos of how room lighting impacts things here: http://www.avintegrated.com/lighting.html
It would be super helpful if you could take some photos of your space, as well as make a drawing (if you don't have one already) of the room and the dimensions so that better information can be provided. Please keep in mind I'm a projector guy first. A speaker guy is a distant second for me. So, listen to what others have to say on that subject.
Make sure either 14 gauge or 12 gauge cabling is pulled to all speaker locations. I usually use 14/4 cabling as it is easier to work with, and I can double up wiring which is nice.
Make sure you have conduit, or a reasonably accessible raceway in place to the projector location for HDMI cabling. HDMI didn't exit 20 years ago, at some point, it won't exist in the future. So, plan for the future by putting in 1.5" flexible conduit from the main equipment location.
Get the equipment out of the room and use a good universal remote to run the show. The heat and noise of equipment should be as removed from the room as possible.
I will think of other things that are worth throwing into the mix before drywall goes up, but those are the first items. I will say that people not doing the lighting right is a huge pet peeve of mine. Wall sconces and LED strip highlights are awesome, but not important to the the room, they are the flair to make it look cool.
FINAL NOTE: I did not read anyone else's responses before I wrote this. There are good recommendations (IMO) about brands like Triad which seem to be well respected for an in-wall speaker that is in a back box to help get audio quality as good as possible. While going to towers up front would be my preference, it's not always possible, and sticking with in-walls for surrounds and ceilings often makes a TON of sense because having those stick out into a room can be problematic in different spaces. So, above recommendations and added information is all spot on.
Give us room dimensions, height, width, depth, along with a few drawings. And tell me what projector you're looking at.