You can check the World of HK section to see the full line of P9S pistols, but most of what you will end up seeing at a gun shop or listed on sites like Gun Broker are either Combat or Target models. These are available in 9mm and .45. The Targets are more valuable than the Combats and the .45s are more valuable than the 9s. As long as these have been out of production, finding a quality example can be a challenge. Often what you find at low prices are either police trade ins or ones that have been rode hard and put away wet.
The Combat models make great carry pieces, especially if you prefer a single stack grip. The Target models are perfect for range work and also make great suppressor hosts, as the tall target sights allow you to see over the suppressor, and adjust quickly for point of aim/impact when switching between suppressed/unsuppressed. Due to the fixed barrels, the P9S pistols are extremely accurate and due to the roller delayed bolt (derived from the G3) they are extremely light in recoil, especially the .45.
Obviously, everyone's budget is different, but since HK no longer repairs these, few other people do either and parts are a challenge to find, my recommendation is that if you want one, be patient, save your money and purchase the best example you can find/afford.
Getting one that come with all of the original kit (spare mag, box, manual, test target and warranty card) makes the entire package more complete and expensive. This is a major perk, if you plan to make this an investment for sale at a later date. Another thing to consider are spare magazines. These are hard to find in quantity or excellent condition. When you do, expect to pay top dollar, so if you can get more than two in a deal, it is generally worth it.