I can tell you this ( as I have a K100D ) but ask for more info in the Pentax DSLR forum..
(1) The K mount on the Pentaxes will probably work OK.
(2) Probably only in what is basically manual way.
(3) Some Ricoh specific lenses have a different set of pins that completely mess up and lock the lens so you cannot unmount ( "the stuck lens problem" ). This can be fixed easily enough. If your lenses are specifically for KR mount this might happen. It can be avoided completely with a simple modification of the lens ( a screwdriver job )..
The A-50mm is probably a good lens, but I don't know the others. I suspect they are not a good enough reason, alone, to stay with Pentax. Unless they are good quality don't use this as a reason is what I'm saying..
Having said that the Pentax K-10D and K100-D owners quite commonly get LBA ( Lens Buying Addiction ) and old manual lenses are quite common purchases. I myself have a 50mm f1.7 manual ( and a darn good lens it is too ), and a 28mm f2.8 ( which I hardly use )..
Bare in mind there is a crop factor of 1.5x, so the field of view of say a 50mm lens will now be like that of a 75mm lens. ( The focal length is unchanged, I gather ). There is actually an upside to this ( apart from longer lenses ) which is that the lens sweet spot now covers more of the frame. This is one reason older lenses that cover the full 35mm frame are popular - they actually behave better than they would on a 35mm camera. There is also the advantage that the K100D, K10D and GX-10 ( Samsung's version of K10D ) all instantly transform all lenses into image stabilized lenses..
I would not knock a Canon, Nikon or Olympus myself as they are all good. So it's a matter of personal choice..
Pentax K100DFuji S5200Fuji E900PCLinuxOS..
Lens design / production has improved a lot over the decades; the only one of your collection that is really any good is the 50mm f/1.7. I have one of these (manual focus and exposure of course) and I like it a lot. It cost 28 on ebay, which is typical, so your lens collection is not exactly a massive investment that you need to protect!..
In your position I'd decide on the basis of the camera and put the old lenses on ebay (apart from the 50mm if you decide to go with pentax). The Pentax K100D is an excellent camera (I have one); so are the Nikon and Canon models you suggest (the Nikon has the better kit lens of those two, b.t.w.). If you want to go with Pentax the K10D is a real bargain at the moment for the quality you get..
But those old lenses aren't one of them. They weren't very good film lenses to start with and I'm pretty sure the Pentax or Olympus kit lenses will out perform them..
Most people would rate the Olympus kit lenses are the best, with Pentax a close second. The Canon kit lenses aren't very good and the Nikon kits are a bit better than Canon but not up to the level of Oly or Pentax..
Note this isn't a judgement on Canon or Nikon optics overall, just on the quality of their kit lenses..
STOP Global Stasis! Change is good!.
Now that you've judged the quality of my typing, take a look at my photos..http://www.photo.net/photos/GlenBarrington..
K100D and K10D both represent probably the best bang for the buck of any DSLRs...
I went thru the same dilemma regarding my Pentax K1000 with 50mm 1.7, and vivitar zooms of 28-70 and 70-210 with 2x converter. But I didnt think I needed the expense or the quality of the more costly digital Pentax K10 or k100..
So sold my stuff to KEH.phography in Atlanta to get a Canon S5. It has the zoom up to 432, the nice flip-and-twist LCD, the good video capability with stereo sound, and the hot shoe which many in it's class do not. S5 is about $350-400.
Plus, through LensMate, I can later get the 2.2X teleconverter lens for about $150 and even the wide angle or macro for about $100 each. Doubt if I will ever need the macro because that portion of the S5 excels. Lensmate has examples of the tele on views of Seattle from 2 miles awayawesome.ngk20000..
I would suggest the Nikon kit lenses are better than the Pentax kit lenses. Vignetting is higher on the Pentax lenses, and the standard Nikon 18-55 is very good..
Pentax K100DFuji S5200Fuji E900PCLinuxOS..
Somehow I think you're missing the point a bit. The OP has made the decision to go for a D-SLR and is now looking for advice on which one to get, telling him to get another compact won't help him much further....
Looking at the PK-lenses you have, I'd say they aren't a good enough reason to choose a Pentax. However, the fact that with the K10D you get a very capable piece of photographic equipment that will give Canon and Nikon a very good run for their money could be a very good reason to choose the K10D. The price in relation to performance is quite good, too..
And if you do get a Pentax, then there are a lot of older and quite cheap lenses out there that would really shine on your new D-SLR. The basic models of both Canon and Nikon are very good, and Olympus also makes some really nice cameras if you like supersmall, light bodies, but if I had to make that choice I'd go for the Pentax, sure enough..
Have fun deciding,PeterThere's no problem so big that you can't run away from it!..