Possibly AquaFish is the cheapest to go if you want to have ready-from-the-box SFOS experience, however the performance of the device is possibly not satisfactory if you want a real zoomer

If you want to go the do-it-yourself way the definite choice is Nexus5 (Hammerhead) since it is very well specced, quite cheap as a second-hand bargain, well supported and has a range of ready-made images available.

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Forgot the question about N9; yes, it is now possible to get SFOS 2.x.x to N9 too, so I have heard but have not tested it myself so cannot say anything about completeness of the build and whether it is usable as a daily driver. The 1.x.x versions had some slight hiccups that would drive all but the most callous users away