Wide advice?
The Sigma 18-35mm f1.8 might be the best option for your purposes, but I've read that some copies of the lens suffer AF issues. You might want to try a copy from somewhere with a generous returns policy. It does give the quality of a fast prime lens throughout it's range though.
[link=https://500px.com/will_brealey/[/link]

I agree the Sigma 18-35 sounds great on paper, but the autofocus issues seem really bad. Doing a quick Google it looks like the Canon guys got a firmware update that improves things, but no such luck for Pentax.
If you like primes, then why not go down this path? If you're worried about light, either increase ISO or do it in Lightroom after the fact (the latter easy if you shoot manual for full control of aperture and shutter speed).
http://www.pbase.com/iberg

f5.6 with bounced flash -

f2.8 with window light only


f3.2 with off camera flash, used as a portrait lens at 50mm

I have owned the DA21mm - I think it's a great street lens, it has a lovely clarity. I found myself wanting something wider though and sold it.
[link=https://500px.com/will_brealey/[/link]

[link=https://500px.com/will_brealey/[/link]
K-1Gripped K-1 ungripped K-5ii K7 Various lenses
Stuart..
I think using on or off camera flash would be a better and cheaper option... Its a great world to explore into....
+1
Peter
My Flickr page
f3.2 with off camera flash, used as a portrait lens at 50mm
Lovely portrait.
Peter
My Flickr page

...... here a tip : (Indoors in extreme low light) - F9.5, ISO 100, 1/180th .... that's how I get both eyes in focus!!
My Guides to the Pentax Digital Camera Flash Lighting System : Download here from the PentaxForums Homepage Article .... link
Pentax K7 with BG-4 Grip / Samyang 14mm f2.8 ED AS IF UMC / DA18-55mm f3.5-5.6 AL WR / SMC A28mm f2.8 / D FA 28-105mm / SMC F35-70 f3.5-4.5 / SMC A50mm f1.7 / Tamron AF70-300mm f4-5.6 Di LD macro / SMC M75-150mm f4.0 / Tamron Adaptall (CT-135) 135mm f2.8 / Asahi Takumar-A 2X tele-converter / Pentax AF-540FGZ (I & II) Flashes / Cactus RF60/X Flashes & V6/V6II Transceiver

I wish you well in the quest for a good 'fast' aperture lens for indoor photography. I do applaud those who can get the conditions right to really achieve quality portraits and people shots with natural light indoors. I do think however it is dependant on many things, quite a few of which are often out of the control of the photographer .... quantity and quality of light being the main ones here. And we have to consider that for each difficulty there is a technical solution, however each solution brings its own particular compromise ... (eg focussing challenges, IQ decrease, DR issues, colour balance problems etc). It is probably a rare situation where it is possible to solve each of those problems at the same time without some significant compromise.
So I think the other guys here were right to point out considering looking at flash as another solution. It has the advantage of being able to solve a number of the 'difficulties' all at once with very little compromise attached.
My Guides to the Pentax Digital Camera Flash Lighting System : Download here from the PentaxForums Homepage Article .... link
Pentax K7 with BG-4 Grip / Samyang 14mm f2.8 ED AS IF UMC / DA18-55mm f3.5-5.6 AL WR / SMC A28mm f2.8 / D FA 28-105mm / SMC F35-70 f3.5-4.5 / SMC A50mm f1.7 / Tamron AF70-300mm f4-5.6 Di LD macro / SMC M75-150mm f4.0 / Tamron Adaptall (CT-135) 135mm f2.8 / Asahi Takumar-A 2X tele-converter / Pentax AF-540FGZ (I & II) Flashes / Cactus RF60/X Flashes & V6/V6II Transceiver
Daronl
Sigma Art 35 F1.4 - The sharpest lens I have ever used on both aps and Ff - really special for all indoors and low light. Always superlative reviews.
Widely reported to have focusing problems....
http://bfy.tw/Hsb5
--
Half Man... Half Pentax ... Half Cucumber
Pentax K-1 + K-5 and some other stuff
Algi
Hypermodern
Member
London
I would like a lens to bridge the gap between my Sigma 30mm f1.4 (old version without HSM, picked up because when my babies became toddlers I needed something faster than the 35mm f2.4 for indoors), and the 10-17 fisheye (so far used mainly for fun architecture and baby photos). I would like something wider than the 30mm but fast enough that I can still use it indoors. Apart from baby photos I do as much opportunistic street as I can whilst pushing the buggy and there is the small matter of my brother's wedding coming up, it's the small registry office bit before the big day and he wants some ceremony/family formals/couple portraits.
I mostly shoot primes, but I'm not averse to a zoom. I do have the 18-135mm but I find it's not fast enough for moving targets indoors. I've been looking at the Sigma 17-50 f2.8 but not sure if this is too big, the equivalent Tamron but maybe this is just the poor cousin of the Sigma, the 21mm Limited but a bit worried about f3.2 indoors, I had thought about the DA 12-24mm but I'm not going to be a landscape photographer any time soon. Help!
tl;dr - need lens for APS-C wider than 30mm, fast enough to shoot moving targets indoors.