Quoted:
On saving money, yes and no.
It depends. My second AR was supposed to be a build. Perusing the gunshow I found it was cheaper to buy a complete bushmaster than it would have been to do the build I had planned. So I bought a complete rifle.

Later builds saved me money, because there were no complete rifles with the features I wanted, so if I had purchased a complete rifle, I'd have ended up with a bunch of spare parts, after replacing them with the components I wanted. It was cheaper to just buy the individual pieces and build from scratch.

What I would suggest, is decide what you want, and what your budget is, and then take a look at what's available on the market and price compare.

Another benefit of building your own is that, if you have the time and patience, you can watch for deals, and buy the pieces as they come up, assemble everything as you go.

Keep in mind that there are some specialized tools that you'll need to buy. And depending on what kind of tools you have available, some regular tools you'll need as well.
In the long run, it's nice to have the AR specific tools around for repairs, or changes. But if you're only buying them to build one rifle, they'll eat up your savings.

Go with a flat-top unless you have a specific reason not too. That is, if you're not trying to build a retro, or KISS rifle, go with the flat-top.

Another piece of advice, don't be in too much of a rush to do anything, read through the technical forums. There are a lot of options, and a lot of manufacturers to choose from, get a real good idea of what you want.