I would call CDNN and see if they have any used Sig P6's left. That was about the best deal on the market for a while. They also might have some .357 Sig HK USPc's for about $499 brand new!
I would be hesitant to recommend a handgun chambered in .357sig to someone as (what sounds to be) a first handgun. A bit spendy and snappy on the recoil.
Ever since he got one, now Alan can stop touting the .357 sig cartridge and rattling off it's benefits. I have to hear it alllllllll day long. He will now likely argue that it is no more expensive or snappy than .40sw among other things.
No EAA Witness, Travis?... :P
Heck yeah! After the few I've shot, I highly recommend any of the CZ clones out there, CZ's, EAA Witness', Armalite AR-24 (supposedly those have some issues), and there are I think several other clones out there. The thing is, most of them seem to be above $400. However, I have seen on Gunbroker that you can find some standard EAA Witness' for under $400, and those are d@mn good guns for the money.
All of that said, considering that you can buy a surplus Sig P6 for ~$260-300 from many retailers, I honestly don't think there is any better deal out there.
One brand that is getting more and more recognition is Bersa. They are great guns, I own three of my own and love 'em
I would suggest that your father in law spend some time at the range with a wide variety of pistols before buying one - or at least handle as many as possible first. A make/model that one person loves might not work for another.
As for value-priced pistols, Glock, Springfield XD, Ruger, Smith & Wesson M&P, and Steyr (if you can find them) are generally considered to be reliable and good values. For revolvers, Ruger offer excellent value, Smith & Wesson makes great revolvers that tend to be pricier, and Taurus seems to be hit-or-miss on finish quality but works OK.
As for caliber, 9mm is readily available from most manufacturers, the cheapest centerfire caliber on the market, has moderate recoil characteristics, and adequate stopping power.
For revolvers, .38 special (.38spl) is close to 9mm in terms of stopping power, with the added bonus that many .38spl revolvers can also chamber the much more potent .357 magnum round.
My father in law is an old timer he has had many rifles, shotguns and revolvers. He just recently decided he would like semi-auto pistol to add to his collection but he just wants one at this time that is reliable but without spending a lot.