I got my K01 recently WITHOUT the DA40XS (price was basically body with a free DAL18-55) but I am still keen on a 40mm to fit.
The following options and B&H prices as follows:
DA40XS:
Pentax DA 40mm f/2.8 XS Lens 22137 B&H Photo Video $246
In terms of sheer smallness and cost obviously the XS gets the nod, especially if bought from a split K01 box. Also arguably has best AF with a K01.
DA40 limited:
Pentax SMCP-DA 40mm f/2.8 Limited Series Autofocus Lens 21550 $496
Love the construction,quick focus, 49mm filter and FF compatibility (I shoot film on occasion), but the much higher price for what is basically the same optic is abit hard to get past.
FA43:
Pentax SMCP-FA 43mm f/1.9 Limited Series Autofocus Lens 20180 $746
Obviously the best of the bunch for the film bodies, best build and still very small compared with my current FA31, this will pretty much live on the K01.
The aperture ring could come in very handy given the single dial of the K01 and is fast and #2 of the 3 legends !
- although quick shift is again missing.
Also of course there are the MF versions of the 40 mm in Pentax and Voightlander mounts but for this exercise I'm after a AF model.
Final points are I am in the market for a FA50 and a MF 50 but once I get those I feel the 40 will stay on camera most of the time saving the prime swapping for my K7.