Hi Peter,
I've been a Pentax user for almost forty years now and they have some very fine glass even in the older series of lenses.
Some of your better options would be to try and find some "A" series lenses as these will still allow auto functions on the K-r, minus the auto focus of course.
For lots of info on everything Pentax have a look
here .
All of the SMC lenses are very good so it depends on how you intend to setup for the macro. There are dedicated macro lenses of course but they will demand a higher price as you may expect but if you intend to use extension tubes the "A" and "M" the "M" being the earlier and completely manual lens and I'd be looking for f/1.7 or faster the f/1.4.
There is even a few f/1.2 but the faster you get the higher the price again of course.
The other option that is often used is to fit a 50mm f/1.7 for example with a reverse adaptor putting the lens on backwards.
This is an example of what can be done with a lens on backwards.
Possibly the most outstanding macro I have ever seen.
if you see anything you want questions answered about let me know and I will possibly have the info or know where to find it.
Ken.