As others have said, while you
can build a bike from scratch cheaply, if you're not careful you can end up spending much more than you'd like. It's great fun though, I loved building mine up.
As for the difference between a DH and and FR bike, I'd say it often comes down more to geometry and setup. Some frames (my wilson included) will have different holes to set it up totally differently for downhill or freeride.
The downhill settings make it longer, slacker, and lowers the BB. Apparently that leads to stability at high speeds and better traction.
The freeride setup shortens the wheelbase raises the BB height, and makes it more responsive at lower speeds.
There can also be a few differences with what the suspension is set up for, with a DH rig the suspension generally aims to keep the wheels on the ground and maximise grip, whereas on an FR bike it's often more to resist bottoming out on big stuff.
I think the biggest problem is the 'freeride' is a very loose term, which just about everyone interprets differently. Some people will use a freeride bike just like a DH bike, and have it set up accordingly, others will be chucking it off massive features in Utah and traction in corners is pretty irrelevant
Some manufactures just sell their DH bike with a single crown fork on the front as a freeride bike, others think the whole bike should be lighter, some heavier. It all depends on what you want to do with it, and your riding style.
Edit: Sorry, didn't realise how much I'd written there, got a bit ramble-y