KM has some very good lenses, so do Pentax.
Nikon and Canon have IS in the lenses, not in the body.
If I were looking to put together a high-end system, even over time, then I would go Nikon or Canon. In fact I use a Canon 20D and am looking to upgrade to a 5D in the reasonably near future.
However for your needs as stated above, the value-for-money of having IS in the body and getting its benefit even with 3rd-party lenses like Sigma makes it very attractive.
That's why I would recommendPentax and KM/Sony for you.