Re: Starting from 0, need some advice
If you use that Roadmaster Granite Peak frame, it will be a tight fit. My bike has almost that same frame, so I can at least say a bit about that one. If you look at the photo of it, the top tube is pretty low near the seat. I needed to use a slant head, as a straight head puts the spark plug too close to the top tube. I also had to offset my carburetor a bit. You will also need an adapter to fit the wide down tube. However, the large frame would give it durability, and having front shocks is extremely nice.
The Glacier Point Magna may be an easier install, as it looks like the down tube is a narrow one, and there's plenty of space. I doubt you would need anything special to make an engine fit that frame.
If you do decide to buy a cheap department store bicycle, inspect it before you buy it. Make sure all welds look good, check for defects, and after you buy it, go through and adjust the brakes and possibly re-pack the wheel bearings with high-quality grease. A crash on a bicycle sucks enough at casual pedalling speed; it's even worse at 30mph with an engine, so you want to make sure nothing will fail.
As far as the engine, there shouldn't be a huge difference between the Mega Motors and the Flying Horse. The Flying Horse is EPA approved and comes from a different factory, but I think they're essentially the same engine. The Flying Horse may have the "new" CNSv2 carburetor, and the Mega Motors may come with the old style NT carburetor, but I cannot say for sure. If you're tight for cash, I don't see a reason why not to just go with the Mega Motors.
I have nothing but good things to say about Bike Berry. They always ship fast, and I've never had a problem with any of their parts. They forgot to send a part once in a large order, but a single email corrected it. If you end up having a problem with your engine, I'm sure they'll be helpful about returns or replacement parts.