Will a Lionel CW-80 be enough to power my 10x7ft layout with Legacy?

This is going to come down to what you plan on running around your loop.

• If you just have a single engine or two with freight cars you're probably going to be ok
• If you have a few engines, smoke, and a bunch of lighted passenger cars then you might see some power fluctuations.

I had a similar loop set up in the past with the Santa Fe PW Celebration set and my CW-80 couldn't handle it. I ended up buying the GW180 set and it was fine.

Just so you know my Lionel 180 powerhouse still powers my Legacy setup today with 3 loops, multiple trains, tons of smoke, lights, etc...

Do I need to by both the GW180 transformer and the 180 Watt Powerhouse or just the 180 Watt Powerhouse to power my layout? 
Can I just buy the TMCC Direct-Lockon and use that with the 180 Watt Powerhouse to power my layout? 

The GW-180 comes with the throttle and the PowerHouse Brick. If you just want the 180 by itself then you can buy only that but you will need a TMCC Direct Lockon to feed the track, or you can make your own adapter because the 180 has a molex type end connector.

• 6-37947 - GW-180 set
• 6-22983 - 180 PowerHouse
• 6-14178 - TMCC Direct Lock-on

I use 6-22983 and 6-14178 in my setup.

I am running a single Legacy steam engine (with main and whistle smoke), a VisionLine reefer car, and a few boxcars with a caboose with the lights shut off. Do you think it will be able to handle it? 

You should be ok, but that's just an educated guess here. Best way to test is to just run them on the track and see if you notice any power loss. Your CW-10 will overload and blink if you do have an issue.