Although these days I most often shoot with a KP, the K-5 IIs, or the FF K-1II, I still have my (mint condition) K20D that I bought new at 1/2 price after the subsequent K-7 came out. It even included a free Pentax 5 year warranty! I still like and still use my K20D at times. Its colors are especially rich. I especially like the results I get when shooting Fall color scenes. Another poster has said he likes it especially for portraits, but I am not sure of the settings he used for this purpose. The K20D tends to underexpose a bit as shown right out of the camera, but I don't bother using the comp control on bright sunny days. This camera has a very fine feel in the hand, and has a very good set of controls. It was the Pentax flagship of its day, very durable and with some important new features over its predecessor.
It would be a good idea, if not already supplied, to download the owners' manual from the Ricoh/pentax website. Also, doing a reset to factory specs in case there have been alterations made by the previous owner.
In order to access any adjustments, or to use the array of controls at your disposal, the mode dial must not be set to the green "auto" mode. This mode bars all access and the camera alone will take charge over all settings, according to what it thinks it "sees" even which Custom Image category it will choose, but does not inform you of its choice. Use the "P" setting instead for full automation of shutter speed and aperture. This setting will allow all access, including the exclusive Pentax Hyper System. For instance, when in automated "P" mode, and the camera sets an aperture or shutter speed not to your liking, you can instantly override the automation by simply using your thumb or finger dial to select your choice and the camera will instantly obey, if available light will allow under the ISO sensitivity you have chosen. You have shifted to Av or Tv operation without having to first turn the mode dial before selecting your desired change. Then, when wished, you can return to full automation with a touch of the green button (or by shutting off the camera). This is the Pentax Hyper Program operating system. Very fast and efficient for making changes on the fly.
The green button can also used when in the Manual mode, which then puts you in Pentax Hyper Manual, which means instead of twirling the thumb and finger dials to set apertures and shutte speeds according to the camera's meter reading, the green button will instantly set meter-centered exposure. This is especially efficient when taking spot-meter readings around a scene. Then, when a satisfactory exposure setting is reached, and you'd rather have a different shutter speed or aperture, first hit the AE-L button to freeze exposure value, then use the thumb or finger dial to make your change, and the other will follow along, preserving that exposure value! The quick and efficient Pentax Hyper Manual System can make the Manual mode of other cameras seem cumbersome by comparison.
Unfortunately, these excellent operational features will not be available with your old manual lenses, nor is across-frame matrix metering, as these old lenses do not have the necessary contacts on their lens mounts to communicate information with the camera's internal workings. I simply want to make you aware for future reference of what advanced and very useful technology sits there before you.
Settings I usually recommend for out of camera imaging- go into the Custom Image menus to set up "Fine Sharpening". Pentax DSLRs, especially flagship models have been verified in lab test reviews to be conservative (a bit softish) in default sharpening. On the K20D, hit the "fn" button, which should bring up the Custom Image menus. You will probably already have the most often used "Bright" category already selected by default. Hit OK which will open this menu. Then the 4 direction arrows around the OK butten will serve for navigation. Tab down to Sharpening adjustments. You will notice in the"Bright" category, sharpening level is already up by one notch from mid-point, which is the default "normal" for this category as it is shown in green. Now you can use your right arrow to move it up one more notch to +2. Then (very important) use your thumb dial to set "Fine Sharpening", which will refine the sharpening process, resulting in better fine detail in your images.
I usually go back up and using the right arrow again, switch to the "Natural" category, which will have less saturated colors, which is often better for skin tones when there are people in your shots. I then set both the sharpening level and "Fine Sharpening" to be the same as in the "Bright" category. I don't do anything in the "Portrait" category.
Your existing old Pentax lenses probably include some primes having larger aperture settings for low light use- especially good with the K20D to keep necessary ISO settings down to a modest level. However, that said, to take full advantage of much this camera can do in good lighting, I suggest you consider the very capable and advanced all-around zoom lens, the Pentax DA 18-135mm DC WR. It features Weather Resistant construction to match that of your K20D, and fast, quiet, accurate DC AF. It provides uncommonly versatile capability and with very fine results. Check out the "DA 18-135mm DC WR lens. show Us What it can do' thread in the SLR lens section. Starting with the most recent entries and go backwards. There will be images taken with it on a variety of newer and older camera bodies with varying results, but you will see many examples that are surprisingly fine for such a wide-ranging zoom lens, even good closeups, and should be a good fit for your K20D's very capable near 15mp resolution. All of your camera's technologies will be available. I'd go for a new lens, as this one can develop zoom creep over time. I carry my cameras in a cross-shoulder & belt loop holster-type case, which is protective and prevents such problems from developing.
Have a good time getting used to and enjoying the considerable potential this fine camera offers!