Routers and Modems

Best Buy is just about the worst place to go to buy computer gear. The prices aren't that great and the sales staff, generally, are not knowledgeable. Then they are very pushy about warranties.

Do you just have a modem from Comcast, their router or are you looking to save money in the longterm by buying your own modem?

Router: A TP-Link AC1200 Gigabit Wireless router should provide you all the bandwidth you need. It offers loads of configuration options. If you want something higher end look at the TP-Link Archer C7.
https://www.amazon.com/TP-Link-Gigabit-Wireless-Archer-C1200/dp/B01IUDUJE0/
https://www.amazon.com/TP-Link-Archer-C7-Wireless-Gigabit/dp/B00BUSDVBQ/

Modem: The TP-Link TC-7610-E will work fine for Comcast and is fast enough for your connection. They also have a 686Mbps model if you want to future proof yourself.
https://www.amazon.com/TP-Link-343Mbps-Certified-Spectrum-TC-7610-E/dp/B010Q29YF8/

If you have a large house. You are better off having WiFi system with distributed Access Points rather than one expensive router. Performance decreases with distance and obstructions. Having a WiFi as close as possible is best.

Assuming you don't have a wired Ethernet network and are not willing/able to run the wires. I would use Powerline adapters. Plug the Ethernet adapter into your router. Then plug the WiFi access point end into another location in your house. If you want to distribute the WiFi more. You just add more WiFi modules. The TP-Link AC1200 Wi-Fi Range Extender is a good unit. Plus, the WiFi modules have Ethernet ports. So, you can wire in a switch or your game consoles and other equipment. For lower latency.
https://www.amazon.com/TP-Link-Extender-Powerline-TL-WPA8630-KIT/dp/B01A90CH4M/

If you do need to add these access points. Use the exact same SSID, Network key and Encryption method. That way your devices will automatically connect to the strongest signal.

If you are can run Ethernet or already have it in place. That is even better than Powerline adapters. Just add additional routers. Do the same wireless settings. Give them static IP address in your network range but outside your DHCP range (so you can locate them for programming purposes). Then disable DHCP so the act as access points and switches.

Agreed that best buy is the worst place to get hardware advice from. They hire salesmen, not tech experts.

For 150mbps connection you will want an 8x4 level modem.
You cant go wrong with a zoom 5345 on amazon for $44 https://www.amazon.com/Zoom-5345-Certified-Spectrum-Providers/dp/B013C4L6OE

For router I have been suggesting the TM-AC1900 for $81.00. This is a special t-mobile branded version of the asus rt-ac68u (a $155 router). It has the same circuit baord (so same dual core cpu, same radios, same memory/storage) as the ac68u just with some t-mobile badges and slightly modified firmware. And if savy enough to flash a different firmware and use putty you can even strip it of the t-mobile firmware, load the original asus one and then load a 3rd party one like asusmerlin.
https://www.amazon.com/T-Mobile-Wireless-AC1900-Dual-Band-AiProtection-Complete/dp/B01MYTAURW/ref=sr_1_1?ie=UTF8&qid=1488209764&sr=8-1&keywords=tm-ac1900
(if you wish to upgrade the firmware buy it from the "ParkDeals" vendor on the amazon website and you can PM me for specific instructions on how to upgrade the firmware).

Are you experiencing some kind of network problem that would warrant upgrading.

Careful of upgrading the modem, check with your local Comcast office to confirm comparability. not saying what will or won't work, just suggesting.

Also just to add:

I agree with using powerline adapters over wifi repeaters.

Do not however agree with using same SSID. Depending on how much overlap you have this could cause a lot of instability. In coroporate level environments they have a server that tells the clients which AP to roam to, without this it is up to the client to decide which AP to connect to. The clients being put in charge of this decision can result in a lot of disconnects due to phantom spikes in power signal tricking the client into thinking it should switch.
So in a home enviroment unless this is avery BIG house or you can tune the power level of the APs very well then I would suggest using different SSIDs

Ur garage issue maybe just the WIFI radio placement. Lag even with 150 mbit, I suggest whatever you get involves some kind of QOS (as usual Google for any terminology not understood) so no one user can monopolize the available bandwidth. $400 for 1 box, it better makes expressos and do me taxes!