You can't replace those lenses you have on FX with limited on the K-5. Sorry, I love the Limited lenses but the Ziess especially do things on FX that no DX/Crop camera can reproduce. (personal opinion of course, but those fast, high micro contrast lenses pop on FX)
That being said. Pentax rocks and if you really just want to step up/down to a smaller lighter body, then do it. Honestly its up to you what you want to do and where you want to take your photography. Personally I take my D700 with me all day everyday. I typically only shoot with a 50/single prime these days and no grip... I don't think its a burden at all.
I think you'll be very happy with Pentax. They make incredible cameras and lenses, my personal favorites are the DA*50-135, DA Limited 77mm, DA Limited 15mm, DA Limtied 35mm, and the FA Limited 31mm. The DA*16-50 gets a lot of hell, but I loved it too. If I were going to pick up a K-5 today I know for a fact I would get the DA*55 and the DA Limited 35mm Macro. I am decidedly more of a prime guy... owning Pentax made me that way
after owning the SMC-M trio I really fell in love with primes and manual focus.
Also, don't forget the same sensor and performance can be had from a D7000 which is of similar size (only saying that if you want to save your lenses).