Take look for some of the older Pentax SMC F lenses they can be found pretty cheap and work well on dslr's, such as the SMC Pentax-F 35-70mm F3.5-4.5. It's a bright really fast focusing lens with decent IQ. I bought a couple that were with Pentax AF Slr's cheap.
Take a look at this one;
http://www.shopgoodwill.com/auctions/Pentax-SF10-35mm-SLR-Camera--Tamron-Lens-16996054.html
I haven't found mine taking the battery down with normal usage much but I have it shut off more for convenience. Of course if you use it and Live View quite often then it uses more battery power, which is another reason I got the Bg4 and a couple of extra batteries just in case.
Tap your info button until the Lcd shuts off. Once you have shut it off via the info button it wont come back on unless you tap the info button. On restart the Lcd will only show the initial settings then go off after a couple of seconds or you can tap the ok button to turn that off immediately . Also you can control the illumination of the top LCD or shut it off in C-4 LCD Panel Illumination.
Another item to consider getting is an ACMaxx Screen protector or something compatible. here's the site but you can purchase it a couple of dollars cheaper on Ebay. They stand by their product is why I like them and it works great.
ACMAXX LCD ARMOR Features http://www.acmaxx.net/pentax_k5_2.html