For the armtube comparison, I would not build a dedicated tonearm from scratch. Using parts from an existing arm is quicker, cheaper and does not introduce new variables and surprises into the test.
An OEM version of Ortofon AS 212 is mechanically simple, is known to sound good, has very few vices and is inexpensive enough to play with. I have a couple, taken from Telefunken S500 turntables. In my previous tonearm experiments I have used them to good effect.
The picture on the right is from the Ortofon page
about this arm.
But AS 212 isn't perfect. Some things are less than optimal. Take geometry: first, its horizontal axis is perpendicular to the armtube, and thus the up-down motion introduces azimuth error. Second, the horizontal axis sits a lot higher than the plane in which the stylus rides. This geometry results in a slight variation of tracking force in relation to the LP groove amplitude. I will write about these geometric problems in a lot more detail later.