If you have a limited budget, this could work, but only if you don't try to play loud music. Also, limit the number of people who have access to the volume and tone controls- I worked in a bar and have done audio work in bars- the one constant is that someone always "knows" they can make it sound better by cranking the tone and volume controls until the speakers and/or amplifier go up in smoke.
If you want it to be more durable, buy some good used equipment, especially the speakers- in a place with those dimensions, there's no way the sound will be too loud in some places and not loud enough in others. The way to get around this is to use a lot more speakers and let them cover the space without being too loud. Not only will the sound quality be better, the acoustics won't cause as many problems.
How high is the ceiling?
The problem with this kind of system is, if you want to use the amplifier in a receiver, is that a receiver can't easily be connected to a large number of speakers without needing something to maintain a usable impedance, losing signal because there's no such thing as controls that don't waste energy and the fact that with any power level, the speakers are sharing it, so a 100W/channel amp that's powering 8 speakers is delivering a whopping 12.5W/speaker, running flat out with no headroom. That won't work.
What will work, is a 70V amplifier and speakers, but you'll need to make sure the power settings are correct if you want the amplifier to survive.
JBL and QSC have a program for determining the number of speakers needed in a space and it allows for choosing their speakers, but you could substitute others, but only if they disperse the sound in the same way as the ones used in the software.
The good news- you can start with a simple system, operate it within its usable range and expand as funds allow by sending the audio out of the REC jacks and connecting it to an amplifier that can provide more power. However, those speakers aren't going to fill a large room with full-range sound. Any speaker made for the rooms in a house will sound tiny if you put them in a large room, or outside. If you want bass, you'll need at least one subwoofer but again, that can be added as funding allows.
Draw the floorplan and think of the speakers output as a 15 foot half-circle, for the purpose of placement and if possible, connect them without mounting, so you can try different locations. You'll have more bass if the speakers are in the corners, especially if the ceiling isn't terribly high. The problem is that the center of the room may not be covered as well as the rest of the seats. DO NOT aim the speakers at the diners- aim over their heads so they aren't in a direct line with the speakers.
