LUKE: "Uncle Owen! This R2 unit has a bad motivator, look!"
The droid uncle Owen wanted to buy at first, nicknamed "Red", was R5-D4, belonging to the R5-series, which was according to the Legends section of its entry in Wookieepedia, "a low cost (...) line, (...) plagued with malfunctions".
According to the same source (Legends via Wookieepedia), the R2-AG-series made use of old R5-series shells when the latter series was discontinued. The R2-AG-series was a line of agromech droids, which sounds much more useful when moisture farming then an astromech droid.
As evidenced by the quote, Luke thought Red to be an R2-series unit.
What did Owen think he was buying?
An R2-AG-series unit, or an R5-series unit? Did the Jawas try to pull a fast one on him, selling an R5 for an R2-AG?
Inspired by the question why the Lars family needed an R2 unit.