Shouldn't you be fixing the rust/mechanicals before spending on a useless mirror?
Yes probably, but even though I haven't done some of the important stuff yet, if I see something that I know I will eventually want come up for sale for a good price, why not snap it up?
I also bought a rust free sunroof panel for £150. They usually go for £300+. I have no intention to put it in yet, but its nice to have it.
Prices of these things will only go up.