If you like the look of Sugar Bear Springers then I would just get one, they are highly regarded for working well, and looking cool too. They ain't cheap, but any good springer that size is gonna be kinda pricey, even really old rusty ones. You can see their version here:
Products | Denvers Choppers
You can also see the approximate rake and frame setup that they are typically used on.
I ordered both my frame and springer from Paughco and was very happy with the quality of both, as have the people who know more than me who have checked out my bike.
"Long bikes" is one of the terms used for choppers like that. They typically have more degrees of rake than other styles of choppers. I would say 40* or 45* would be what you'd see.
One thing you can do is buy a cheap clear plastic protractor at the office supply store and use it to measure rake on bikes. You just overlay it on a profile (side view) of the bike (hopefull at or near 90*) and line the straight edge up with the level bottom of the tires and read the rake angle right off it. If you do this with a few bikes you like the looks of you'll probably figure out how much rake you want.
The other two frame measures that are important are stretch, which are both measured in inches. The two stretches are "up" and "out". "Up" stretch is measure of how much higher the top of the head tube is above the stock location, and "out" stretch is the measure of how far forward of the stock position the head tube is.
People who like low and long bikes, like the digger style often do something like 3" out, 0" up. The "Denver Chopper" look (like I said sometimes called "long bikes") has quite a bit of both up and out, as you can see from the pictures on the link above. Paughco lets you take their basic frame and specify some of these dimensions (or pick between a large number of variations, actually).
My bike is a big-twin, not a Sportster, so I used a different frame than you would. I wanted it low and compact, but with aggressive rake. I got a 0" up, 0" out 40* rake frame.
Getting the frame level, I agree that was really important to me. I didn't know a lot so here is what I did. It took a long time, I'm sure there is a smarter way to do this, but it will work and give you success.
I bought the frame I wanted, bought the wheels I wanted and mounted the tires on them. Then I put the rear wheel on, and put blocks of wood under the frame until it had the stance I wanted. Then I took a long piece of wood and put it through the head tube (where the forks would go) and measured the distance between the bottom of the head tube and the wheel axle. I called up Paughco and asked them which springer would result in that distance being what I wanted, and was told how to figure that (and determined which springer would duplicate the length of wood measurement.)
I ordered that, and it fit perfectly and gave me just the stance I wanted. This was slow, because you order, wait and then measure, order and wait again. But I was (am) not in a hurry and was more concerned with getting my >$1000 springer order correct.