1. at 17mm and 50mm, u hardly need any stabilisation on a bright day lah :dunno: shutter speed can go > 1/200... i leave it on most of the time unless i'm using tripod. i'm indifferent to it. but i can go as low as 1/8 on 50mm if i hold my breath. ymmv. i think it also helps that pentax dslrs are relatively lighter so its less tiring to hold it.
2. depends what u want to achieve. the effect with flash and without flash is very different. there are some shots where it is better to bump up ISO (say, up to 3200?) and these newer models can certainly deliver relatively clean shots at that ISO. there are also some shots that can benefit from using a flash (i.e. u want to step down for more DOF).
there had always been complaints that pentax's p-ttl is not very accurate. i actually find it ok... sure, may get some misses when i bounce but i hasn't failed me catastrophically so far...

i'm using an old metz 48. the newer model (metz 50) have better build quality (metal foot) and is worth a look. i find metz to be better value for money than the first party models. but some prefer first party cos controls are more straightforward. switches vs LCD screen.