Originally posted by Adam The D FA 28-105mm is definitely the way to go for your walkaround lens. It's waaaay better than any legacy budget zoom I've tested so far (including the FA 28-105mm F4-5.6).
The FA 50mm is good, as is the DA 50mm if you don't mind a hint of vignetting and softer corners.
As a telephoto zoom, the DA* 60-250mm works surprisingly well in FF mode.
I'd also recommend the D FA 100mm as a dual-purpose telephoto/macro lens.
I hate to post "me too!" messages, but I couldn't agree more. Some of the official sample photos released by Pentax are shot with the D FA 28-105mm, and it looks fantastic (all things considered).
The Tamron SP AF 70-200/2.8 can be had really cheaply (I got mine used on eBay for about $400 or so), and is quite good in terms of sharpness (though bokeh is a bit nervous).
The Tamron 90/2.8 macro hovers around in the sub-$200 range on eBay now and then -- it's very sharp.
There's obviously a ton of old Pentax 300/4 and 300/4.5 manual-focus lenses on eBay. Most people find these acceptably sharp on APS-C bodies, so they should definitely be sharp enough on full-frame.
I know I sound like a broken record, but one beautiful thing about moving from a 24 MP APS-C camera to a 36 MP FF camera is that the larger pixel size will be a lot less taxing on older lenses;
any lens that you'd consider center-sharp on a 24 megapixel APS-C camera should appear even sharper in the center on 36 megapixel FF. I'm obviously not saying anything about vignetting or corner sharpness; those vary widely based on the lens design.