I hav'ent got, nor tried the DA40, but the images I have seen, it is a very impressive lens.
Now to the DA35 and FA50. I have both, and both are exceptional.

The 35mm is Pentax's little secret. A "cheap" lens that is stunning! It is oh so sharp, and produces an excellent creamy bokeh. Not quite in the FA31 Limited league(what is?) but so close it hurts, and it is less than a fifth of the price of the mighty 31mm.

Now the FA50. Some say it is not as good as the F1.7 version, but it depends on your taste. However, it is the only Pentax nifty still available new, so we can discount the F1.7. F1.4 is addictive, a tad soft, but creative. However it is more or less the same price as the DA40, but is two stops faster. ON the plus side, if you still have Film SLR's then it will be suitable for these due to having the aperture ring (my MZ-5n needs the A ring on a lens.). Downside is that on APS-C DSLR's, it is a little bit long, having an EFOV of 75mm.

Horses for courses really, all three lenses are excellent, and will produce stunning results, so it all comes down to budget and what you want to use it for.
35mm is a great general purpose lens
40mm is a fine tiny Limited, which will get you noticed (what is that small lens? how wierd is that!)
50mm is super fast, and great for portraits.
So, whichever you go for, you will get a great lens.

I had a look at the brisca site, and it is like 30 years ago. Stu Smith jr v Frankie Wainman Jr v Craig Finnikin (Bert's son?) v Paul Harrison (Willies Son?) V Danny Wainman.
I can't Believe John Lund is still racing though, he must be at least 65 now.

Stock Cars. Excellent, I used to go a lot many years ago, Belle Vue (old Stadium, White City, Bradford, etc etc. The days of Stu Smith, Frankie Wainman, John Lund.
Yes, I think the 120-400 will be excellent for it.

I went to a motorcross event at Mallory Park with togging friends of mine. I took my 170-500 and 70-300 cheapo Sigma as my longest lenses. Most of the time the 170-500 was far too long, even at 170mm. So in the end I used my 70-300 instead, with my 50-135 for some close to the track shots.
With hindsight, I think the DA* 60-250 would be perfect for 80% of the shots, but not having one of those, I coped with the different combos.

As an aside, one of my friends there, who does a lot of motorsport work with his Canon 1D mk3, and being a true blood canonite (nothing else will match etc etc) hired a Sigma 120-300 F2.8, used it hand held, and thought it perfect for motorsport.
So, if you are very flush with cash, wait for the mk2 120-300 from Sigma on the Pentax mount, which has been confirmed. If not, then either the 60-250 or the Sigma 70-200 will probably be more than suitable most of the time. Or, if you are looking at a budget lens, then even the 70-300 OS or 55-300 Pentax will be great, as you will be shooting at 1/125 F11 anyway.
Hope this helps.

I take it you have got the 120-400mm then? Good choice. Mind you the 150-500 is just as good.

flat4. I have the 170-500 and because it is an older design, I have found it struggling to keep up with my K-5. It is better on my K20D. Looks like I will be getting either the 120-400 or 150-500 in the not too distant future.

I agree with the OP. It is the one thing that I prefer on my K20D to my K-5. The Instant SR switch was a boon when you are constantly taking the camera on and off the tripod as I was today with the K-5. Having to go through the menus is a pain.

Can't agree more. Zooms have been getting better and better and are very useful as you just stand there and adjust the focal length to suit.
The Downside is that they do make you very lazy.
A prime, however, makes you think more, makes you move, makes you compose a little bit more. And of course, primes are so much faster than a zoom.
Since I have got my 35mm, It has made me think more, and move more. It's like my 50mm on my film camera.
Zooms are not dead, far from it, but prime use is on the rise as you work harder, but the results are so much more satifying as a result.

Mark,

The Sigma 105mm Macro is a stunning lens. I have one and is very sharp.
Comparing it to my DA*50-135mm at the eqv focal length and aperture, there is not much difference. The DA* probably just edges it, but only because I consider it the finest Portrait zoom out there. However, for Macro, no contest
If it is a good price, go for it. You will get an excellent lens.

Good choice with the 10-20 and 17-70 sigmas. The former has a cult following and nice and sharp. The Latter I have not used, but from accounts it is a great lens.

Now to the primes.

DA 35mm F2.4 is a great start point, quite fast and very sharp. Also very light and will be great on the K-7 K-5 bodies that you have shortlisted.

As for another lens, the fifty's are beginning to go up in price, mainly due to the fact that a new FA 50mm F1.4 is now quite steep in price.
So, I would suggest a fully manual (ie no A on the aperture ring or AF) 55mm F1.8. It may be old but is very sharp, and currently a bargain, you are talking about £40-50 at the moment.

Hope this helps.

It won't be plastic. The K-5 is magnesium Alloy, so will be more durable

However, I have no held one yet. Maybe Focus in March?