Retirement looms, ... I have had a Pentax ist DSLR for a few years, and have decided to purchase a K7 body, but have no clue which lens I should get. ....
My wife said the same thing to me, and I came up with the same thought, however my camera was a 35 year old Spotmatic II. So, I started out with a K100. Personally, I would bypass the K7 and start on lenses, for several reasons. The ist is perfectly fine to continue to learn on, and in fact probably much better than the K7. After a while, you may grow out of the ist and something new will be out . That is the time to get the current top of the line body. In the meantime, pick up the lenses that interest you most, and glass is a much better investment than bodies. Build around the current lens you have on the ist - probably the kit lens, a 18-55, I am guessing. I have noticed that there are a lot of folks still using the ist. Yes, you hear and read posts from folks here that have the latest widgets. But lots of fine photography is still done with the ist - it is not by any means an inferior camera.
Yes - the all-in-one lens, 18-250 is nice - a single lens and easy. The primes are nice, and you can concentrate on the areas that interest you most, wide or medium or telephoto. The idea of picking up some inexpensive lenses would be a great learning and cheap way in to the hobby - especially the older M42 screw on lenses (needs a K adapter), but they are wonderful pieces of glass. You learn apertures and manual focusing - and time really is probably not an issue. Then you can pick up better glass as time goes on, plus you will know what focal lengths you are really interested in.
I personally like to do landscapes. I really do not need fast lenses (expensive) and can easily get by with f4 lenses. If I need light, I use a tripod and take a longer exposure.
In your post you say "but have no clue which lens I should get". As noted before, part of photography is having a set of tools that fit the task - or in this case the type of images that you enjoy best. Its somewhat a-kin to going into a hardware store and saying I need a saw. There are finish saws, rip saws, cross cut saws, miter box saws, coping saw, etc. each for a specific purpose tuned to a particular job. Sure they will all cut wood, but using a coping saw to rip a 2x4 is pretty tough.
So, to answer your question, I am guessing that you have a kit lens, 18-55. For general photography, I would supplement that with a 50-200 and would pick it up from the market place below
You can usually get them for about $100. Then you have a pair of lenses that range from wide angle to telephoto. Using them to experiment to see what you like to shoot most - birds, flowers, landscapes, a little bit of everything, the grand kids, kids in the park playing baseball, etc. Then, build a lens collection around what you like to shoot most. Along the way, maybe pickup an inexpensive manual 50 f2 (should be no more than $20). Great for indoors and night photos of the sky.
So, I am still working, which limits the time I can shoot. So, I usually take advantage of sunsets and early evening - low light, so that can be difficult and challenging. After about 4 years now, I finally upgraded to a K20 right when the K7 was coming out. The price was half the K7 and I was able to get all the features that I felt I needed. K7 would be nice, however the K8 or K9 will be even better, as I run in the limitations of the K20 - which will be some where way out in the future.
Don't think of retirement as "looming".
You should welcome it and embrace it with open arms.
It gives you the time and freedom to do those things you always wanted to do.
You will be free of deadlines and commitments and obligations and time clocks - virtually all of those encumbrances that have been so limiting.
After 11 years of retirement I can honestly say I have never been bored.
My photography occupies a great deal of my time as do my several other interests.
I truly loved my work and I did miss it for a while but must admit that ................... Life begins with retirement.