I've got a fairly "budget" setup that covers my garage/shop as well as backyard/deck areas, and I've had quite a few buddies **** me into helping them get systems setup after partying at my place... this is one of the few topics I actually have solid experience in.
First and foremost I'd steer clear of using a home theater/surround sound amplifier, instead go with a two channel commercial amplifier used off eBay/craiglist/facebook. I'm partial to QSC but Crown also makes some great units. In my setup I run a QSC GXD 4 1600 watt 2 channel amplifier. I paid under $200 for mine, it's able to handle my 9 speakers running at 8-ohm impedance with ease. I've got mine setup to run split channels, Channel 1 is connected to a long range bluetooth receiver, I've used several but currently I'm using a 1Mii. Any reciever that's BT 5.0, and has aptX is good, check Amazon there's quite a few available. Channel 1 is for streaming from phones/laptops/anything bluetooth enabled (or connected to a BT adapter), while channel 2 is connected to a remote source switcher which can switch between my shop computer, cable box, xbox/PS4, etc...
Now as far as connecting speakers to the amp you have a few routes you can go down, if you want control centrally you can run something like Theater Solutions TS8D Eight Zone Dual Source Selector which offers circuit protection and impedance matching as well as an easy way to turn off certain speaker pairs (or more, if running in parallel). You can also run a the output into a single source distribution block and from there connect to something like OSD's resistive volume controls (they come in quite a few different flavors, resistive, passive, 100 watt or 300 watt, a-b selectable or not..). The idea is that you want to try and keep your impedance consistent 8 ohm, 4 ohm, 16 ohm (depending on the amp you get and how you lay your setup out) so that you don't have volume spikes and don't stress the amp unnecessarily. So long as you have impedance control devices along the way your only limit to number of speakers is going to be power output...
As far as speakers, I'll echo the others in this thread... don't do in-ceiling unless you absolutely must. bookshelf/boxed speakers mounted up high will provide a far better experience. I'm a Klipsch fanboy (I'm not gonna lie, the copper cones are like 50% of what I like about them... they're so pretty!) and I've had great luck tracking down scratch and dent/cosmetically challenged speakers for my shop on the cheap. I have 3 floor standing towers along my shop's back wall that were $350 each at retail, but due to water damage on the bottom and heavy scratches I paid $200... they still sound great, and I don't have to feel guilty about having them in the shop. It's a similar story with the 4 smaller bookshelf speakers i have mounted up high at the front of the shop, they're not pretty but they sound great. The outdoor speakers I bought off monoprice and they also seem to work quite well.
For only 650sq ft I'd think something like a QSC GX5 or even GX3 amp and 4-6 speakers (depending on if you go with floor standing towers or smaller bookshelf speakers). Off the top of my head with the amp at $200, wire at $100, and volume/channel/impedance control at $250-$300 you've got a solid $400 to blow on speakers. The bigger they are and the more dynamic range they have, the better!
Right, ok I think I've rambled on about this for long enough for now... if you want specifics on any of that wall of text above feel free to ask, I can certainly elaborate (more...).